U.S. patent number 9,704,869 [Application Number 15/238,343] was granted by the patent office on 2017-07-11 for semiconductor memory device having an electrically floating body transistor.
This patent grant is currently assigned to Zeno Semiconductor, Inc.. The grantee listed for this patent is Zeno Semiconductor, Inc.. Invention is credited to Zvi Or-Bach, Yuniarto Widjaja.
United States Patent |
9,704,869 |
Widjaja , et al. |
July 11, 2017 |
Semiconductor memory device having an electrically floating body
transistor
Abstract
An IC may include an array of memory cells formed in a
semiconductor, including memory cells arranged in rows and columns,
each memory cell may include a floating body region defining at
least a portion of a surface of the memory cell, the floating body
region having a first conductivity type; a buried region located
within the memory cell and located adjacent to the floating body
region, wherein the buried region has a second conductivity type,
wherein the floating body region is bounded on a first side by a
first insulating region having a first thickness and on a second
side by a second insulating region having a second thickness, and a
gate region above the floating body region and the second
insulating region and is insulated from the floating body region by
an insulating layer; and control circuitry configured to provide
electrical signals to said buried region.
Inventors: |
Widjaja; Yuniarto (San Jose,
CA), Or-Bach; Zvi (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Zeno Semiconductor, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Zeno Semiconductor, Inc. (San
Jose, CA)
|
Family
ID: |
45889725 |
Appl.
No.: |
15/238,343 |
Filed: |
August 16, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160358924 A1 |
Dec 8, 2016 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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14930049 |
Nov 2, 2015 |
9450090 |
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14330083 |
Jul 14, 2014 |
9208840 |
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14018947 |
Sep 5, 2013 |
8817548 |
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12897516 |
Oct 4, 2010 |
8547756 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C
7/22 (20130101); H01L 29/66825 (20130101); G11C
11/4099 (20130101); G11C 11/39 (20130101); G11C
11/404 (20130101); G11C 14/0018 (20130101); H01L
29/66833 (20130101); H01L 29/7841 (20130101); G11C
11/4074 (20130101); G11C 11/4094 (20130101); H01L
27/1023 (20130101); H01L 29/7881 (20130101); H01L
27/10802 (20130101); H01L 29/0821 (20130101); H01L
23/528 (20130101); H01L 29/772 (20130101); G11C
11/4096 (20130101); H01L 29/1004 (20130101); H01L
29/1095 (20130101); G11C 2211/4016 (20130101); G11C
11/4026 (20130101); G11C 11/04 (20130101) |
Current International
Class: |
G11C
7/00 (20060101); H01L 29/10 (20060101); G11C
7/22 (20060101); H01L 23/528 (20060101); G11C
11/4094 (20060101); G11C 11/4074 (20060101); H01L
27/108 (20060101); G11C 11/404 (20060101); H01L
29/78 (20060101); H01L 29/772 (20060101); G11C
11/04 (20060101) |
Field of
Search: |
;365/189.011,189.04 |
References Cited
[Referenced By]
U.S. Patent Documents
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|
Primary Examiner: Hoang; Huan
Attorney, Agent or Firm: Venable LLP Schwarz; Steven J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 14/930,049, filed Nov. 2, 2015, which is a continuation of U.S.
patent application Ser. No. 14/330,083, filed Jul. 14, 2014, which
is a continuation of U.S. patent application Ser. No. 14/018,947,
filed Sep. 5, 2013, which is a continuation of U.S. patent
application Ser. No. 12/897,516, filed Oct. 4, 2010. This
application is also commonly assigned with U.S. patent application
entitled "COMPACT SEMICONDUCTOR MEMORY DEVICE HAVING REDUCED NUMBER
OF CONTACTS, METHODS OF OPERATING AND METHODS OF MAKING," Ser. No.
12/897,528, filed on Oct. 4, 2010. The entire contents of all the
foregoing applications are incorporated herein in their entirety by
reference.
Claims
What is claimed is:
1. A semiconductor memory cell formed in a semiconductor, the
semiconductor memory cell comprising: a floating body region
defining at least a portion of a surface of the semiconductor
memory cell, the floating body region having a first conductivity
type; and a buried region located within the semiconductor memory
cell and located adjacent to the floating body region, wherein the
buried region has a second conductivity type, and the buried region
is discontinuous along one direction; wherein said floating body
region stores a charge or lack of charge indicative of a state of
the semiconductor memory cell selected from at least first and
second states; and wherein said buried region is configured to
generate impact ionization when the semiconductor memory cell is in
one of said first and second states, and wherein said buried region
is configured so as not to generate impact ionization when the
semiconductor memory cell is in the other of said first and second
states.
2. The semiconductor memory cell of claim 1, further comprising a
bit line region at said surface, the bit line region having the
second conductivity type.
3. The semiconductor memory cell of claim 2, further comprising a
single shared contact through which bit line regions of adjacent
memory cells are coupled.
4. The semiconductor memory cell of claim 2, further comprising a
common region through which bit line regions of adjacent cells are
coupled, the common region having the second conductivity type.
5. The semiconductor memory cell of claim 1, further comprising a
gate above said surface and insulated from said surface by an
insulating layer.
6. The semiconductor memory cell of claim 1, further comprising a
first well region of the first conductivity type beneath the buried
region.
7. The semiconductor memory cell of claim 1, wherein said buried
region is adapted to receive electrical signals of different
amplitude or polarity, and wherein said electrical signals depend
on an operation of said semiconductor memory cell.
8. An array of memory cells formed in a semiconductor, the array
comprising: a plurality of memory cells arranged in a plurality of
rows and a plurality of columns, each of said memory cells
comprising: a floating body region defining at least a portion of a
surface of the memory cell, the floating body region having a first
conductivity type; and a buried region located beneath the surface
of the memory cell, wherein the buried region has a second
conductivity type, wherein the rows of the plurality of memory
cells define a first direction and the columns of the plurality of
memory cells define a second direction, and said buried region is
discontinuous along the first direction or the second direction and
commonly connected to at least two of said memory cells, and when a
first memory cell of said plurality of memory cells is in a first
state and a second memory cell of said plurality of memory cells is
in a second state, application of a bias to said buried region
maintains said first memory cell in said first state and said
second memory cell in said second state.
9. The array of memory cells of claim 8, wherein each of said
memory cells further comprises a bit line region located in the
floating body region and substantially exposed at said surface, the
bit line region having the second conductivity type.
10. The array of memory cells of claim 9, further comprising a
plurality of source lines crossing the array in a first direction
beneath said surface, wherein the plurality of source lines are
coupled to said buried regions.
11. The array of memory cells of claim 10, further comprising a
plurality of bit lines crossing the array in a second direction
substantially orthogonal to the first direction, wherein the
plurality of bit lines are coupled at said surface to said bit line
regions.
12. The array of memory cells of claim 8, wherein each of said
plurality of memory cells further comprises a gate region above
said surface and insulated from said surface by an insulating
layer.
13. The array of memory cells of claim 8, wherein each of said
plurality of memory cells further comprises a first well region of
the first conductivity type beneath the buried region.
14. The array of memory cells of claim 8, wherein said buried
region is adapted to receive electrical signals of different
amplitude or polarity, wherein the electrical signals depend on an
operation of each of said memory cells.
15. The array of memory cells of claim 8, further comprising a
voltage generator circuitry configured to apply a voltage to said
buried regions.
16. An array of memory cells formed in a semiconductor, the array
comprising: a plurality of memory cells arranged in a plurality of
rows and a plurality of columns, each of said memory cells
comprising: a floating body region defining at least a portion of a
surface of the memory cell, the floating body region having a first
conductivity type; and a buried region located beneath the surface
of the memory cell, wherein the buried region has a second
conductivity type, wherein said floating body region stores a
charge or lack of charge indicative of a state of the semiconductor
memory cell selected from at least first and second states; and
wherein the rows of the plurality of memory cells define a first
direction and the columns of the plurality of memory cells define a
second direction, and said buried region is discontinuous along the
first direction or the second direction and commonly connected to
at least two of said plurality of memory cells, wherein said buried
region is configured to generate impact ionization when the memory
cell is in one of said first and second states, and wherein said
buried region is configured so as not to generate impact ionization
when the memory cell is in the other of said first and second
states.
17. The array of memory cells of claim 16, wherein each of said
memory cells further comprises a bit line region located in the
floating body region and substantially exposed at said surface, the
bit line region having the second conductivity type.
18. The array of memory cells of claim 17, further comprising a
plurality of source lines crossing the array in a first direction
beneath said surface, wherein the plurality of source lines are
coupled to said buried region.
19. The array of memory cells of claim 18, further comprising a
plurality of bit lines crossing the array in a second direction
substantially orthogonal to the first direction, wherein the
plurality of bit lines are coupled at said surface to said bit line
region.
20. The array of memory cells of claim 16, wherein said buried
region is adapted to receive electrical signals of different
amplitude or polarity, wherein the electrical signals depend on an
operation of each of said memory cells.
Description
TECHNICAL FIELD
The present invention relates to semiconductor memory technology.
More specifically, the present invention relates to a semiconductor
memory device having an electrically floating body transistor.
BACKGROUND OF THE INVENTION
Semiconductor memory devices are used extensively to store data.
Static and Dynamic Random Access Memory (SRAM and DRAM) are widely
used in many applications. SRAM typically consists of six
transistors and hence has a large cell size. However, unlike DRAM,
it does not require periodic refresh operation to maintain its
memory state. Conventional DRAM cells consist of one-transistor and
one-capacitor (1T/1C) structure. As the 1T/1C memory cell features
are scaled, difficulties arise due to the necessity of maintaining
the capacitance value.
DRAM based on the electrically floating body effect has been
proposed (see for example "A Capacitor-less 1T-DRAM Cell", S.
Okhonin et al., pp. 85-87, IEEE Electron Device Letters, vol. 23,
no. 2, February 2002 ("Okhonin-1"), which is incorporated by
reference herein in its entirely and "Memory Design Using
One-Transistor Gain Cell on SOT", T. Ohsawa et al., pp. 152-153,
Tech. Digest, 2002 IEEE International Solid-State Circuits
Conference, February 2002) ("Ohsawa-1"), which is incorporated by
reference herein in its entirely. Such a memory eliminates the
capacitor used in conventional 1T/1C memory cell, and thus is
easier to scale to smaller feature size. In addition, such memory
allows for a smaller cell size compared to the conventional 1T/1C
memory cell. Both Okhonin-1 and Ohsawa-1 describe DRAM memory cell
comprising a single standard metal-oxide-semiconductor field effect
transistor (MOSFET) having a gate terminal, two source/drain
terminals, and a floating body fabricated using
silicon-on-insulator (SOT) complimentary metal-oxide-semiconductor
(CMOS) technology. Oshawa-1 further describes a current mirror
sense amplifier which compares the current of a sensed cell to the
average of two reference cells, one written to logic-0 and the
other written to logic-1.
In a floating body memory, the different memory states are
represented by different levels of charge in the floating body. In
Okhonin-1 and Ohsawa-1, a single bit (two voltage levels) in a
standard MOSFET is contemplated. Others have described using more
than two voltage levels stored in the floating body of a standard
MOSFET allowing for more than a single binary bit of storage in a
memory cell like, for example, "The Multistable Charge-Controlled
Memory Effect in SOT Transistors at Low Temperatures", Tack et al.,
pp. 1373-1382, IEEE Transactions on Electron Devices, vol. 37, May
1990 ("Tack") which is incorporated by reference herein in its
entirely, and U.S. Pat. No. 7,542,345 "Multi-bit memory cell having
electrically floating body transistor, and method of programming
and reading same" to Okhonin, et al ("Okhonin-2"). Tack describes
obtaining more than two states in the floating body of a standard
MOSFET built in SOT by manipulating the "back gate"--a conductive
layer below the bottom oxide (BOX) of the silicon tub the MOSFET
occupies. Okhonin-2 discloses attaining more than two voltage
states in the floating body utilizing the intrinsic bipolar
junction transistor (BJT) formed between the two source/drain
regions of the standard MOSFET to generate read and write
currents.
In memory design in general, sensing and amplifying the state of a
memory cell is an important aspect of the design. This is true as
well of floating body DRAM memories. Different aspects and
approaches to performing a read operation are known in the art
like, for example, the ones disclosed in "A Design of a
Capacitor-less 1T-DRAM Cell Using Gate-Induced Drain Leakage (GIDL)
Current for Low-power and High-speed Embedded Memory", Yoshida et
al., pp. 913-918, International Electron Devices Meeting, 2003
("Yoshida") which is incorporated by reference herein in its
entirely; in U.S. Pat. No. 7,301,803 "Bipolar reading technique for
a memory cell having an electrically floating body transistor"
("Okhonin-3") which is incorporated by reference herein in its
entirely; and in "An 18.5 ns 128 Mb SOI DRAM with a Floating Body
Cell", Ohsawa et al., pp. 458-459, 609, IEEE International
Solid-State Circuits Conference, 2005 ("Ohsawa-2") which is
incorporated by reference herein in its entirely. Both Yoshida and
Okhonin-3 disclose a method of generating a read current from a
standard MOSFET floating body memory cell manufactured in SOI-CMOS
processes. Okhonin-3 describes using the intrinsic BJT transistor
inherent in the standard MOSFET structure to generate the read
current. Ohsawa-2 discloses a detailed sensing scheme for use with
standard MOSFET floating body memory cells implemented in both SOI
and standard bulk silicon.
Writing a logic-0 to a floating body DRAM cell known in the art is
straight forward. Either the source line or the bit line is pulled
low enough to forward bias the junction with the floating body
removing the hole charge, if any. Writing a logic-1 typically may
be accomplished using either a band-to-band tunneling method (also
known as Gate Induced Drain Leakage or GIDL) or an impact
ionization method
In floating body DRAM cells, writing a logic-0 is straightforward
(simply forward biasing either the source or drain junction of the
standard MOSFET will evacuate all of the majority carriers in the
floating body writing a logic-0) while different techniques have
been explored for writing a logic-1. A method of writing a logic-1
through a gate induced band-to-band tunneling mechanism, as
described for example in Yoshida. The general approach in Yoshida
is to apply an appropriately negative voltage to the word line
(gate) terminal of the memory cell while applying an appropriately
positive voltage to the bit line terminal (drain) and grounding the
source line terminal (source) of the selected memory cell. The
negative voltage on WL terminal and the positive voltage on BL
terminal creates a strong electric field between the drain region
of the MOSFET transistor and the floating body region in the
proximity of the gate (hence the "gate induced" portion of GIDL) in
the selected memory cell. This bends the energy bands sharply
upward near the gate and drain junction overlap region, causing
electrons to tunnel from the valence band to the conduction band,
leaving holes in the valence band. The electrons which tunnel
across the energy band become the drain leakage current (hence the
"drain leakage" portion of GIDL), while the holes are injected into
floating body region 24 and become the hole charge that creates the
logic-1 state. This process is well known in the art and is
illustrated in Yoshida (specifically Figs. 2 and 6 on page 3 and
Fig. 9 on page 4).
A method of writing a logic-1 through impact ionization is
described, for example, in "A New 1T DRAM Cell with Enhanced
Floating Body Effect", Lin and Chang, pp. 23-27, IEEE International
Workshop on Memory Technology, Design, and Testing, 2006, ("Lin")
which is incorporated in its entirety by reference herein. The
general approach in Lin is to bias both the gate and bit line
(drain) terminals of the memory cell to be written at a positive
voltage while grounding the source line (source). Raising the gate
to a positive voltage has the effect of raising the voltage
potential of the floating body region due to capacitive coupling
across the gate insulating layer. This in conjunction with the
positive voltage on the drain terminal causes the intrinsic n-p-n
bipolar transistor (drain (n=collector) to floating body (p=base)
to source (n=emitter)) to turn on regardless of whether or not a
logic-1 or logic-0 is stored in the memory cell. In particular, the
voltage across the reversed biased p-n junction between the
floating body (base) and the drain (collector) will cause a small
current to flow across the junction. Some of the current will be in
the form of hot carriers accelerated by the electric field across
the junction. These hot carriers will collide with atoms in the
semiconductor lattice which will generate hole-electron pairs in
the vicinity of the junction. The electrons will be swept into the
drain (collector) by the electric field and become bit line
(collector) current, while the holes will be swept into the
floating body region, becoming the hole charge that creates the
logic-1 state.
Much of the work to date has been done on SOI, which is generally
more expensive than a bulk silicon process. Some effort has been
made to reduce costs of manufacturing floating body DRAMs by
starting with bulk silicon. An example of a process to selectively
form buried isolation region is described in "Silicon on
Replacement Insulator (SRI) Floating Body Cell (FBC) Memory", S.
Kim et al., pp. 165-166, Tech Digest, Symposium on VLSI Technology,
2010, ("S_Kim") which is incorporated in its entirety by reference
herein. In S_Kim bulk silicon transistors are formed. Then the
floating bodies are isolated by creating a
silicon-on-replacement-insulator (SRI) structure. The layer of
material under the floating body cells is selectively etched away
and replaced with insulator creating an SOI type of effect. An
alternate processing approach to selectively creating a gap and
then filling it with an insulator is described in "A 4-bit Double
SONOS Memory (DSM) with 4 Storage Nodes per Cell for Ultimate
Multi-Bit Operation", Oh et al., pp. 58-59, Tech Digest, Symposium
on VLSI Technology, 2006 ("Oh") which is incorporated in its
entirety by reference herein.
Most work to date has involved standard lateral MOSFETs in which
the source and drain are disposed at the surface of the
semiconductor where they are coupled to the metal system above the
semiconductor surface. A floating body DRAM cell using a vertical
MOSFET has been described in "Vertical Double Gate Z-RAM technology
with remarkable low voltage operation for DRAM application", J. Kim
et al., pp. 163-164, Symposium of VLSI Technology, 2010, ("J_Kim")
which is incorporated in its entirety by reference herein. In
J_Kim, the floating body is bounded by a gate on two sides with a
source region above and a buried drain region below. The drain is
connected to a tap region, which allows a connection between a
conductive plug at the surface to the buried drain region.
An alternate method of using a standard lateral MOSFET in a
floating body DRAM cell is described in co-pending and commonly
owned U.S. Patent Application Publication 2010/0034041 to Widjaja
("Widjaja"), which is incorporated in its entirety by reference
herein. Widjaja describes a standard lateral MOSFET floating body
DRAM cell realized in bulk silicon with a buried well and a
substrate which forms a vertical silicon controlled rectifier (SCR)
with a P1-N2-P3-N4 formed by the substrate, the buried well, the
floating body, and the source (or drain) region of the MOSFET
respectively. This structure behaves like two bipolar junction
transistor (BJT) devices coupled together--one an n-p-n (N2-P3-N4)
and one a p-n-p (P3-N2-P1)--which can be manipulated to control the
charge on the floating body region (P3).
The construction and operation of standard MOSFET devices is well
known in the art. An exemplary standard metal-oxide-semiconductor
field effect transistor (MOSFET) device 100 is shown in FIG. 52A.
MOSFET device 100 consists of a substrate region of a first
conductivity type 82 (shown as p-type in the figure), and first and
second regions 84 and 86 of a second conductivity type (shown as
n-type) on the surface 88, along with a gate 90, separated from the
semiconductor surface region by an insulating layer 92. Gate 90 is
positioned in between the regions 84 and 86. Insulating layers 96
can be used to separate one transistor device from other devices on
the silicon substrate 82.
As shown in FIG. 52B, a standard MOSFET device 100A may also
consist of a well region 94A of a first conductivity type (shown as
p-type in the figure) in a substrate region 82A of a second
conductivity type (shown as n-type in the figure), with first and
second regions 84A and 86A of a second conductivity type on the
surface 88A. In addition, a gate 90A, separated from the surface
region 88A by an insulating layer 92A, is also present in between
the first and second regions 84A and 86A. Insulating layers 96A can
be used to separate one transistor device from other devices in the
well region 94A. MOSFET devices 100 and 100A are both constructed
in bulk silicon CMOS technology.
As shown in FIG. 52C, a standard MOSFET device 100B is shown
constructed out of silicon-on-insulator technology. MOSFET device
100B consists of a tub region of a first conductivity type 82B
(shown as p-type in the figure), and first and second regions 84B
and 86B of a second conductivity type (shown as n-type) on the
surface 88B, along with a gate 90B, separated from the
semiconductor surface region by an insulating layer 92B. Gate 90B
is positioned in between the regions 84B and 86B. The tub region
82B is isolated from other devices on the sides by insulating
layers 96B and on the bottom by insulating layer 83B. Optionally,
there may be a conductive layer affixed to the bottom of insulating
layer 83B (not shown) which may be used as a "back gate" by
coupling through the insulating layer 83B to the tub region
82B.
The transistors 100, 100A, and 100B are all called n-channel
transistors because when turned on by applying an appropriate
voltage to the gates 90, 90A and 90B respectively, the p-material
under the gates is inverted to behave like n-type conductivity type
for as long as the gate voltage is applied. This allows conduction
between the two n-type regions 84 and 86 in MOSFET 100, 84A and 86A
in MOSFET 100A and 84B and 86B in MOSFET 100B. As is well known in
the art, the conductivity types of all the regions may be reversed
(i.e., the first conductivity type regions become n-type and the
second conductivity type regions become p-type) to produce
p-channel transistors. In general, n-channel transistors are be
preferred for use in memory cells (of all types and technologies)
because of the greater mobility of the majority carrier electrons
(as opposed to the majority carrier holes in p-channel transistors)
allowing more read current for the same sized transistor, but
p-channel transistors may be used as a matter of design choice.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A through 1E illustrate an array and details of a first
exemplary memory cell according to the present invention.
FIGS. 2A through 2U illustrate a method of manufacturing a memory
cell according to the present invention.
FIGS. 3A through 3C illustrate a method of maintaining the state of
a memory cell according to the present invention.
FIGS. 4A through 4D illustrate methods of maintaining the state of
the data stored in an array of memory cells according to the
present invention.
FIG. 5 is a graph of the floating body voltage in a memory cell
according to the present invention.
FIG. 6 is a graph of current-voltage curves of a memory cell
according to the present invention.
FIG. 7 illustrates a read operation performed on an array of memory
cells according to the present invention.
FIGS. 8A through 8H illustrate the operation of four representative
memory cells of the array of FIG. 7.
FIGS. 9A and 9B illustrates the operation of selected memory cells
according to the present invention during a first type of write
logic-0 operation.
FIG. 10 illustrates an array of memory cells according to the
present invention during the first type of write logic-0 operation
of FIG. 9.
FIGS. 11A and 11B illustrate the operation of unselected memory
cells according to the present invention of the array of FIG. 10
during a first type of write logic-0 operation.
FIG. 12 illustrates an array of memory cells according to the
present invention during a second type of write logic-0
operation.
FIG. 13 illustrates an array of memory cells according to the
present invention during a third type of write logic-0
operation.
FIGS. 14A through 14H illustrate the operation of four
representative memory cells of the array of FIG. 13 during the
third type of logic operation.
FIG. 15 illustrates an array of memory cells according to the
present invention during a first type of write logic-1
operation.
FIGS. 15A through 15H illustrate the operation of four
representative memory cells of the array of FIG. 15 during the
first type of write logic-1 operation.
FIG. 16 illustrates an array of memory cells according to the
present invention during a second type of write logic-1
operation.
FIGS. 16A through 16H illustrate the operation of four
representative memory cells of the array of FIG. 16 during the
second type of write logic-1 operation.
FIGS. 17A through 17E illustrate a second exemplary memory cell
according to the present invention.
FIGS. 18A through 18H illustrate performing operations on an array
of the memory cell of FIGS. 17A through 17E.
FIGS. 19A through 19F illustrate multilevel operations on a memory
cell according to the present invention.
FIG. 20 illustrates an alternate method of manufacturing a memory
cell according to the present invention.
FIG. 21 illustrates a top view of the memory cell of FIG. 20.
FIG. 22A illustrates another alternate method of manufacturing a
memory cell according to the present invention.
FIG. 22B illustrates an array of the memory cell of FIG. 22A.
FIGS. 23A through 23F illustrates a third exemplary memory cell
according to the present invention.
FIGS. 24A through 24F illustrate an alternate physical embodiment
of the memory cell of FIGS. 23A through 23F.
FIG. 25A illustrates an array of the memory cell of the embodiments
of FIGS. 23A through 23F and FIGS. 24A through 24F.
FIG. 25B illustrates a circuit schematic of an individual cell of
the embodiments of FIGS. 23A through 23F and FIGS. 24A through
24F.
FIG. 26 illustrates a hold operation performed on the array of FIG.
25A.
FIG. 27 illustrates a read operation performed on the array of FIG.
25A.
FIGS. 28A through 28P illustrate the operation of eight
representative memory cells of the array of FIG. 27.
FIG. 29 illustrates a two row write logic-0 operation on the memory
array of FIG. 25A.
FIGS. 29A and 29B illustrate the operation of unselected memory
cells in FIG. 29.
FIG. 30 illustrates a single column write logic-0 operation on the
memory array of FIG. 25A.
FIG. 31 illustrates a single memory cell write logic-0 operation on
the memory array of FIG. 25A.
FIGS. 32A through 32P illustrate the operation of eight
representative memory cells of the array of FIG. 31.
FIG. 33 illustrates a single memory cell write logic-1 operation on
the memory array of FIG. 25A.
FIGS. 34A through 34P illustrate the operation of eight
representative memory cells of the array of FIG. 33.
FIG. 35 illustrates an alternate single memory cell write logic-1
operation on the memory array of FIG. 25A.
FIGS. 36A through 36B illustrates a possible write disturb
condition resulting from the single memory cell write logic-1
operation of FIG. 35.
FIG. 37 illustrates another alternate single memory cell write
logic-1 operation on the memory array of FIG. 25A.
FIGS. 38A and 38B illustrates additional alternate methods of
manufacturing a memory cell according to the present invention.
FIGS. 39A through 39AA illustrate a method of manufacturing the
memory cell of FIG. 38B.
FIGS. 40A through 40F illustrate a fourth exemplary memory cell
according to the present invention.
FIGS. 41A and 41B illustrate different holding operations on a
memory array of the memory cells of FIGS. 40A through 40F.
FIGS. 42 and 42A through 42H illustrate a read operation on a
memory array of the memory cells of FIGS. 40A through 40F.
FIG. 43 illustrates a single memory cell write logic-0 operation on
the memory array of FIG. 25A.
FIGS. 44A through 44B illustrate the operation of the unselected
memory cells of the array of FIG. 43.
FIG. 45 illustrates a single memory cell write logic-0 operation on
the memory array of FIG. 25A.
FIGS. 46A through 46H illustrate the operation of four
representative memory cells of the array of FIG. 45.
FIGS. 47A through 47F illustrate a fifth exemplary memory cell
according to the present invention.
FIG. 48 illustrates the hold operation when using memory cells of
the present invention in SCR mode.
FIG. 49 illustrates the single cell read operation when using
memory cells of the present invention in SCR mode.
FIG. 50 illustrates the single cell write logic-1 operation when
using memory cells of the present invention in SCR mode.
FIG. 51 illustrates the single cell write logic-0 operation when
using memory cells of the present invention in SCR mode.
FIGS. 52A through 52C illustrate standard MOSFET transistors of the
prior art.
DETAILED DESCRIPTION OF THE INVENTION
The invention below describes a semiconductor memory device having
an electrically floating body that utilizes a back bias region to
further reduce the memory device size. One or more bits of binary
information may be stored in a single memory cell. Methods of
construction and of operation of the semiconductor device are also
provided.
This disclosure uses the standard convention that p-type and n-type
semiconductor "diffusion" layers or regions (regardless of how
formed during manufacture) such as transistor source, drain or
source/drain regions, floating bodies, buried layers, wells, and
the semiconductor substrate as well as related insulating regions
between the diffusion regions (like, for example, silicon dioxide
whether disposed in shallow trenches or otherwise) are typically
considered to be "beneath" or "below" the semiconductor
surface--and the drawing figures are generally consistent with this
convention by placing the diffusion regions at the bottom of the
drawing figures. The convention also has various "interconnect"
layers such as transistor gates (whether constructed of metal,
p-type or n-type polysilicon or some other material), metal
conductors in one or more layers, contacts between diffusion
regions at the semiconductor surface and a metal layer, contacts
between the transistor gates and a metal layer, vias between two
metal layers, and the various insulators between them (including
gate insulating layers between the gates and a diffusion at the
semiconductor surface) are considered to be "above" the
semiconductor surface--and the drawing figures are generally
consistent with this convention placing these features, when
present, near the top of the figures. One exception worth noting is
that gates may in some embodiments be constructed in whole or in
part beneath the semiconductor surface. Another exception is that
some insulators may be partially disposed both above and below the
surface. Other exceptions are possible. Persons of ordinary skill
in the art will appreciate that the convention is used for ease of
discussion with regards to the standard way of drawing and
discussing semiconductor structures in the literature, and that a
physical semiconductor in use in an application may be deployed at
any angle or orientation without affecting its physical or
electrical properties thereby.
The exemplary embodiments disclosed herein have at most one surface
contact from the semiconductor region below the semiconductor
surface to the interconnect region above the semiconductor surface
within the boundary of the memory cell itself. This is in contrast
to one-transistor (1T) floating body cell (FBC) DRAMs of the prior
art which have two contacts--one for the source region and one for
the drain region of the transistor. While some 1T FBC DRAM cells of
the prior art can share the two contacts with adjacent cells
resulting in an average of one contact per cell, some embodiments
of the present invention can also share its contact with an
adjacent cell averaging half a contact per cell.
The advantage of the present invention is in the elimination of one
of the source/drain regions at the surface of the semiconductor
region thereby eliminating the need to contact it at the surface.
Compare, for example, FIG. 52B illustrating a prior art MOSFET with
FIG. 1C illustrating a analogous cross section of one embodiment of
the present invention. In any processing technology, the structure
of FIG. 1C is inherently smaller than the structure of FIG. 52B. In
some embodiments of the present invention, the gate terminal is
removed as well further reducing the size of the memory cell.
Compare, for example, the analogous cross sections of the
structures in FIGS. 40C and 47C to the prior art MOSFET of FIG.
52B. This new class of memory cell is referred to as a "Half
Transistor Memory Cell" as a convenient shorthand for identical,
similar or analogous structures. A structure identical, similar or
analogous to the structure of FIG. 1C is referred to as a "Gated
Half Transistor Memory Cell." A structure identical, similar or
analogous to the structures of FIGS. 40C and 47C is referred to as
a "Gateless Half Transistor Memory Cell." The vertical arrangement
of the diffusion regions beneath the semiconductor surface common
to all half transistor memory cells--specifically a bit line region
at the surface of the semiconductor (allowing coupling to a bit
line disposed above the semiconductor surface), a floating body
region (for storing majority charge carriers, the quantity of
majority carriers determining the logical state of the data stored
in memory cell), and a source line region (completely beneath the
semiconductor surface within the boundary of the memory cell
allowing coupling to a source line running beneath the
semiconductor surface, typically running beneath and coupling to a
plurality of memory cells), wherein the bit line region, the
floating body, and the source line region form a vertical bipolar
junction transistor that is used operatively and constructed
deliberately by design for use in a floating body DRAM memory cell
application--is referred to as a "Half Transistor."
Persons of ordinary skill in the art will appreciate that the
following embodiments and methods are exemplary only for the
purpose of illustrating the inventive principles of the invention.
Many other embodiments are possible and such alternate embodiments
and methods will readily suggest themselves to such skilled persons
after reading this disclosure and examining the accompanying
drawing. Thus the disclosed embodiments are exemplary only and the
present invention is not to be limited in any way except by the
appended claims.
Drawing figures in this specification, particularly diagrams
illustrating semiconductor structures, are drawn to facilitate
understanding through clarity of presentation and are not drawn to
scale. In the semiconductor structures illustrated, there are two
different conductivity types: p-type where the majority charge
carriers are positively charged holes that typically migrate along
the semiconductor valence band in the presence of an electric
field, and n-type where the majority charge carriers are negatively
charged electrons that typically migrate along the conduction band
in the presence of an electric field. Dopants are typically
introduced into an intrinsic semiconductor (where the quantity of
holes and electrons are equal and the ability to conduct electric
current is low: much better than in an insulator, but far worse
than in a region doped to be conductive--hence the "semi-" in
"semiconductor") to create one of the conductivity types.
When dopant atoms capable of accepting another electron (known and
"acceptors") are introduced into the semiconductor lattice, the
"hole" where an electron can be accepted becomes a positive charge
carrier. When many such atoms are introduced, the conductivity type
becomes p-type and the holes resulting from the electrons being
"accepted" are the majority charge carriers. Similarly, when dopant
atoms capable of donating another electron (known and "donors") are
introduced into the semiconductor lattice, the donated electron
becomes a negative charge carrier. When many such atoms are
introduced, the conductivity type becomes n-type and the "donated"
electrons are the majority charge carriers.
As is well known in the art, the quantities of dopant atoms used
can vary widely over orders of magnitude of final concentration as
a matter of design choice. However it is the nature of the majority
carries and not their quantity that determines if the material is
p-type or n-type. Sometimes in the art, heavily, medium, and
lightly doped p-type material is designated p+, p and p-
respectively while heavily, medium, and lightly doped n-type
material is designated n+, n and n- respectively. Unfortunately,
there are no precise definitions of when a "+" or a "-" is an
appropriate qualifier, so to avoid overcomplicating the disclosure
the simple designations p-type and n-type abbreviated "p" or "n"
respectively are used without qualifiers throughout this
disclosure. Persons of ordinary skill in the art will appreciate
that there are many considerations that contribute to the choice of
doping levels in any particular embodiment as a matter of design
choice.
Numerous different exemplary embodiments are presented. In many of
them there are common characteristics, features, modes of
operation, etc. When like reference numbers are used in different
drawing figures, they are used to indicate analogous, similar or
identical structures to enhance the understanding of the present
invention by clarifying the relationships between the structures
and embodiments presented in the various diagrams--particularly in
relating analogous, similar or identical functionality to different
physical structures.
FIGS. 1A through 1E illustrate an embodiment of a gated half
transistor FBC DRAM memory cell according to the present invention.
FIG. 1A shows a top view of an embodiment of a partial memory array
including memory cell 50 (shown by a dotted line) and FIG. 1B shows
memory cell 50 in isolation. FIGS. 1C and 1D show the memory cell
50 cross sections along the I-I' line and II-II' cut lines,
respectively, while FIG. 1E shows a method for electrically
contacting the buried well and substrate layers beneath the
cell.
Referring to FIGS. 1C and 1D together, the cell 50 includes a
substrate 12 of a first conductivity type such as a p-type, for
example. Substrate 12 is typically made of silicon, but may also
comprise, for example, germanium, silicon germanium, gallium
arsenide, carbon nanotubes, or other semiconductor materials. In
some embodiments of the invention, substrate 12 can be the bulk
material of the semiconductor wafer. In other embodiments,
substrate 12 can be a well of the first conductivity type embedded
in either a well of the second conductivity type or, alternatively,
in the bulk of the semiconductor wafer of the second conductivity
type, such as n-type, for example, (not shown in the figures) as a
matter of design choice. To simplify the description, the substrate
12 will usually be drawn as the semiconductor bulk material as it
is in FIGS. 1C and 1D.
A buried layer 22 of a second conductivity type such as n-type, for
example, is provided in the substrate 12. Buried layer 22 may be
formed by an ion implantation process on the material of substrate
12. Alternatively, buried layer 22 can also be grown epitaxially on
top of substrate 12.
A floating body region 24 of the first conductivity type, such as
p-type, for example, is bounded on top by bit line region 16 and
insulating layer 62, on the sides by insulating layers 26 and 28,
and on the bottom by buried layer 22. Floating body 24 may be the
portion of the original substrate 12 above buried layer 22 if
buried layer 22 is implanted. Alternatively, floating body 24 may
be epitaxially grown. Depending on how buried layer 22 and floating
body 24 are formed, floating body 24 may have the same doping as
substrate 12 in some embodiments or a different doping, if desired
in other embodiments, as a matter of design choice.
Insulating layers 26 and 28 (like, for example, shallow trench
isolation (STI)), may be made of silicon oxide, for example, though
other insulating materials may be used. Insulating layers 26 and 28
insulate cell 50 from neighboring cells 50 when multiple cells 50
are joined in an array 80 to make a memory device as illustrated in
FIGS. 4A-4C. Insulating layer 26 insulates both body region 24 and
buried region 22 of adjacent cells (see FIG. 1C), while insulating
layer 28 insulates neighboring body region 24, but not the buried
layer 22, allowing the buried layer 22 to be continuous (i.e.
electrically conductive) in one direction (along the II-II'
direction as shown in FIG. 1D). This connecting of adjacent memory
cells together through buried layer 22 forming a source line
beneath adjacent memory cells 50 allows the elimination of a
contacted source/drain region or an adjacent contacted plug inside
the memory cell required in memory cells of the prior art. As can
be seen in FIGS. 1A and 1B, there is no contact to the buried layer
22 at the semiconductor surface inside the boundary of memory cell
50.
A bit line region 16 having a second conductivity type, such as
n-type, for example, is provided in floating body region 24 and is
exposed at surface 14. Bit line region 16 is formed by an
implantation process formed on the material making up substrate 12,
according to any implantation process known and typically used in
the art. Alternatively, a solid state diffusion process could be
used to form bit line region 16.
A gate 60 is positioned in between the bit line region 16 and
insulating layer 26 and above the floating body region 24. The gate
60 is insulated from floating body region 24 by an insulating layer
62. Insulating layer 62 may be made of silicon oxide and/or other
dielectric materials, including high-K dielectric materials, such
as, but not limited to, tantalum peroxide, titanium oxide,
zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 60
may be made of, for example, polysilicon material or metal gate
electrode, such as tungsten, tantalum, titanium and their
nitrides.
Cell 50 further includes word line (WL) terminal 70 electrically
connected to gate 60, bit line (BL) terminal 74 electrically
connected to bit line region 16, source line (SL) terminal 72
electrically connected to buried layer 22, and substrate terminal
78 electrically connected to substrate 12.
As shown in FIG. 1E, contact between SL terminal 72 and buried
layer 22 can be made through region 20 having a second conductivity
type, and which is electrically connected to buried well region 22,
while contact between substrate terminal 78 and substrate region 12
can be made through region 21 having a first conductivity type, and
which is electrically connected to substrate region 12.
The SL terminal 72 connected to the buried layer region 22 serves
as a back bias terminal, i.e. a terminal at the back side of a
semiconductor transistor device, usually at the opposite side of
the gate of the transistor coupled to the body or bulk of the
device corresponding to region 82 in transistor 100 of FIG. 52A or
region 94A in transistor 100A in FIG. 52B. In a floating body DRAM
cell, a conductive coupling to the floating body would be
counterproductive since it would cease to be a floating body with
such a connection. In some embodiments, the p-n junction between
the floating body 24 and the buried well 22 coupled to the source
line terminal 72 is forward biased to be conductive by applying a
negative voltage to the source line terminal 72. In some
embodiments, the SL terminal is biased to a positive voltage
potential to maintain the charge in the floating body region 24. In
some embodiments, the source line terminal 72 is used in a manner
similar to the source line in floating body DRAM cells of the prior
art. Thus in various embodiments SL terminal 72 may be used in a
manner similar to a back bias terminal, or it may be used like a
source line, or it may be used for another purpose entirely. In
some embodiments it may be used in two or more of these ways in
different operations. Thus both the terms "source line terminal"
and "back bias terminal" are used interchangeably in this
specification and should be deemed equivalent.
Comparing the structure of the memory device 50, for example, as
shown in FIG. 1C to the structure of transistor devices 100, 100A
and 100B in FIGS. 52A through 52C, it can be seen that the memory
device of present invention constitutes a smaller structure
relative to the MOSFET devices 100, 100A and 100B, where only one
region of a second conductivity type is present at the surface of
the silicon substrate. Thus, memory cell 50 of the present
invention provides an advantage that it consists of only one region
of second conductivity at the surface (i.e. bit line region 16 as
opposed to regions 84 and 86 or regions 84A and 86A) and hence
requires only one contact per memory cell 50 (i.e. to create a
connection between bit line region 16 and terminal 74).
Persons of ordinary skill in the art will appreciate that in FIGS.
1A through 1E and that the first and second conductivity types can
be reversed in memory cell 50 as a matter of design choice and that
the labeling of regions of the first conductivity type as p-type
and the second conductivity type as p-type is illustrative only and
not limiting in any way. Thus the first and second conductivity
types can be p-type and n-type respectively in some embodiments of
memory cell 50 and be n-type and p-type respectively in other
embodiments. Further, such skilled persons will realize that the
relative doping levels of the various regions of either
conductivity type will also vary as a matter of design choice, and
that there is no significance to the absence of notation signifying
higher or lower doping levels such as p+ or p- or n+ or n- in any
of the diagrams.
A method of manufacturing memory cell 50 will be described with
reference to FIGS. 2A through 2U. These 21 figures are arranged in
groups of three related views, with the first figure of each group
being a top view, the second figure of each group being a vertical
cross section of the top view in the first figure of the group
designated I-I', and the third figure of each group being a
horizontal cross section of the top view in the first figure of the
group designated II-II'. Thus FIGS. 2A, 2D, 2G, 2J, 2M, 2P and 2S
are a series of top views of the memory cell 50 at various stages
in the manufacturing process, FIGS. 2B, 2E, 2H, 2K, 2N, 2Q and 2T
are their respective vertical cross sections labeled I-I', and
FIGS. 2C, 2F, 2I, 2L, 2O, 2R and 2U are their respective horizontal
cross sections labeled II-II'. Identical reference numbers from
FIGS. 1A through 1G appearing in FIGS. 2A through 2U represent
similar, identical or analogous structures as previously described
in conjunction with the earlier drawing figures. Here "vertical"
means running up and down the page in the top view diagram and
"horizontal" means running left and right on the page in the top
view diagram. In a physical embodiment of memory cell 50, both
cross sections are vertical with respect to the surface of the
semiconductor device.
Turning now to FIGS. 2A through 2C, the first steps of the process
are seen. In an exemplary 130 nanometer (nm) process a thin silicon
oxide layer 102 with a thickness of about 100 A may be grown on the
surface of substrate 12. This may be followed by a deposition of
about 200 A of polysilicon layer 104. This in turn may be followed
by deposition of about 1200 A silicon nitride layer 106. Other
process geometries like, for example, 250 nm, 180 nm, 90 nm, 65 nm,
etc., may be used. Similarly, other numbers of, thicknesses of, and
combinations of protective layers 102, 104 and 106 may be used as a
matter of design choice.
As shown in FIGS. 2D through 2F, a pattern opening the areas to
become trench 108 may be formed using a lithography process. Then
the silicon oxide 102, polysilicon 104, silicon nitride 106 layers
may be subsequently patterned using the lithography process and
then may be etched, followed by a silicon etch process, creating
trench 108.
As shown in FIGS. 2G through 2I, a pattern opening the areas to
become trenches 112 may be formed using a lithography process,
which may be followed by etching of the silicon oxide 102,
polysilicon 104, silicon nitride layers 106, and a silicon trench
etch process, creating trench 112. The trench 112 is etched such
that the trench depth is deeper than that of trench 108. In an
exemplary 130 nm process, the trench 108 depth may be about 1000 A
and the trench 112 depth may be about 2000 A. Other process
geometries like, for example, 250 nm, 180 nm, 90 nm, 65 nm, etc.,
may be used. Similarly, other trench depths may be used as a matter
of design choice.
As shown in FIGS. 2J through 2L, this may be followed by a silicon
oxidation step, which will grow silicon oxide films in trench 108
and trench 112 which will become insulating layers 26 and 28. In an
exemplary 130 nm process, about 4000 A silicon oxide nay be grown.
A chemical mechanical polishing step can then be performed to
polish the resulting silicon oxide films so that the silicon oxide
layer is flat relative to the silicon surface. A silicon dry
etching step can then be performed so that the remaining silicon
oxide layer height of insulating layers 26 and 28 may be about 300
A from the silicon surface. In other embodiments the top of
insulating layers 26 and 28 may be flush with the silicon surface.
The silicon nitride layer 106 and the polysilicon layer 104 may
then be removed which may then be followed by a wet etch process to
remove silicon oxide layer 102 (and a portion of the silicon oxide
films formed in the area of former trench 108 and former trench
112). Other process geometries like, for example, 250 nm, 180 nm,
90 nm, 65 nm, etc., may be used. Similarly, other insulating layer
materials, heights, and thicknesses as well as alternate sequences
of processing steps may be used as a matter of design choice.
As shown in FIGS. 2M through 2O, an ion implantation step may then
be performed to form the buried layer region 22 of a second
conductivity (e.g. n-type conductivity). The ion implantation
energy is optimized such that the buried layer region 22 is formed
shallower than the bottom of the insulating layer 26 and deeper
than the bottom of insulating layer 28. As a result, the insulating
layer 26 isolates buried layer region 22 between adjacent cells
while insulating layer 28 does not isolate buried layer region 22
between cells. This allows buried layer region 22 to be continuous
in the direction of the II-II' cross section. Buried layer 22
isolates the eventual floating body region 24 of the first
conductivity type (e.g., p-type) from the substrate 12.
As shown in FIGS. 2P through 2R, a silicon oxide or high-dielectric
material gate insulation layer 62 may then be formed on the silicon
surface (e.g. about 100 A in an exemplary 130 nm process), which
may then be followed by a polysilicon or metal gate 60 deposition
(e.g. about 500 A in an exemplary 130 nm process). A lithography
step may then be performed to pattern the layers 62 and 60, which
may then be followed by etching of the polysilicon and silicon
oxide layers. Other process geometries like, for example, 250 nm,
180 nm, 90 nm, 65 nm, etc., may be used. Similarly, other gate and
gate insulation materials with different thicknesses may be used a
matter of design choice.
As shown in FIGS. 2S through 2U, another ion implantation step may
then be performed to form the bit line region 16 of a second
conductivity type (e.g. n-type conductivity). This may then be
followed by backend process to form contact and metal layers (not
shown in FIGS. 2A through 2U). The gate 60 and the insulating
layers 26 and 28 serve as masking layer for the implantation
process such that regions of second conductivity are not formed
outside bit line region 16. In this and many subsequent figures,
gate layer 60 and gate insulating layer 62 are shown flush with the
edge of insulating layer 26. In some embodiments, gate layer 60 and
gate insulating layer 62 may overlap insulating layer 16 to prevent
any of the implant dopant for bit line region 16 from inadvertently
implanting between gate layer 60 and gate insulating layer 62 and
the adjacent insulating layer 26.
The states of memory cell 50 are represented by the charge in the
floating body 24. If cell 50 is positively charged due to holes
stored in the floating body region 24, then the memory cell will
have a lower threshold voltage (the gate voltage where an ordinary
MOSFET transistor is turned on--or in this case, the voltage at
which an inversion layer is formed under gate insulating layer 62)
compared to if cell 50 does not store holes in body region 24.
The positive charge stored in the floating body region 24 will
decrease over time due to the diode leakage current of the p-n
junctions formed between the floating body 24 and bit line region
16 and between the floating body 24 and the buried layer 22 and due
to charge recombination. A unique capability of the invention is
the ability to perform the holding operation in parallel to all
memory cells of the array.
As shown in FIG. 3A, the holding operation can be performed by
applying a positive back bias to buried layer 22 through the SL
terminal 72 while simultaneously grounding the bit line region 16
through the BL terminal 74 and grounding the substrate 12 through
substrate terminal 78. The positive back bias applied to the buried
layer region connected to the SL terminal will maintain the state
of the memory cell 50 that it is connected to. The holding
operation is relatively independent of the voltage applied to gate
60 through word line terminal 70. In some embodiments of the
invention, the word line terminal may be grounded. Inherent in the
memory cell 50 is n-p-n bipolar device 30 formed by buried well
region 22 (the collector region), floating body 24 (the base
region), and bit line region 16 (the emitter region).
If floating body 24 is positively charged, a state corresponding to
logic-1, the bipolar transistor 30 formed by bit line region 16,
floating body 24, and buried well region 22 will be turned on due
to an impact ionization mechanism like that described with
reference to Lin cited above. In particular, the voltage across the
reversed biased p-n junction between the floating body 24 and the
buried well region 22 will cause a small current to flow across the
junction. Some of the current will be in the form of hot carriers
accelerated by the electric field across the junction. These hot
carriers will collide with atoms in the semiconductor lattice which
will generate hole-electron pairs in the vicinity of the junction.
The electrons will be swept into the buried layer region 22 by the
electric field, while the holes will be swept into the floating
body region 24.
The hole current flowing into the floating region 24 (usually
referred to as the base current) will maintain the logic-1 state
data. The efficiency of the holding operation can be enhanced by
designing the bipolar device formed by buried well region 22,
floating region 24, and bit line region 16 to be a low-gain bipolar
device, where the bipolar gain is defined as the ratio of the
collector current flowing out of SL terminal 72 to the base current
flowing into the floating region 24.
FIG. 3B shows the energy band diagram of the intrinsic n-p-n
bipolar device 30 when the floating body region 24 is positively
charged and a positive bias voltage is applied to the buried well
region 22. The dashed lines indicate the Fermi levels in the
various regions of the n-p-n transistor 30. The Fermi level is
located in the band gap between the solid line 17 indicating the
top of the valance band (the bottom of the band gap) and the solid
line 19 indicating the bottom of the conduction band (the top of
the band gap) as is well known in the art. The positive charge in
the floating body region lowers the energy barrier of electron flow
into the base region. Once injected into the floating body region
24, the electrons will be swept into the buried well region 22
(connected to SL terminal 72) due to the positive bias applied to
the buried well region 22. As a result of the positive bias, the
electrons are accelerated and create additional hot carriers (hot
hole and hot electron pairs) through an impact ionization
mechanism. The resulting hot electrons flow into the SL terminal 72
while the resulting hot holes will subsequently flow into the
floating body region 24. This process restores the charge on
floating body 24 and will maintain the charge stored in the
floating body region 24 which will keep the n-p-n bipolar
transistor 30 on for as long as a positive bias is applied to the
buried well region 22 through SL terminal 72.
If floating body 24 is neutrally charged (the voltage on floating
body 24 being equal to the voltage on grounded bit line region 16),
a state corresponding to logic-0, no current will flow through the
n-p-n transistor 30. The bipolar device 30 will remain off and no
impact ionization occurs. Consequently memory cells in the logic-0
state will remain in the logic-0 state.
FIG. 3C shows the energy band diagram of the intrinsic n-p-n
bipolar device 30 when the floating body region 24 is neutrally
charged and a bias voltage is applied to the buried well region 22.
In this state the energy level of the band gap bounded by solid
lines 17A and 19A is different in the various regions of n-p-n
bipolar device 30. Because the potential of the floating body
region 24 and the bit line region 16 is equal, the Fermi levels are
constant, resulting in an energy barrier between the bit line
region 16 and the floating body region 24. Solid line 23 indicates,
for reference purposes, the energy barrier between the bit line
region 16 and the floating body region 24. The energy barrier
prevents electron flow from the bit line region 16 (connected to BL
terminal 74) to the floating body region 24. Thus the n-p-n bipolar
device 30 will remain off.
The difference between an impact ionization write logic-1 operation
as described with reference to Lin cited above and a holding
operation is that during a holding operation the gate 60 is not
biased at a higher voltage than normal during a holding operation.
During a write logic-1 operation, the capacitive coupling from the
gate 60 to the floating body region 24 forces the n-p-n bipolar
device 30 on regardless of the data stored in the cell. By
contrast, without the gate boost a holding operation only generates
carriers through impact ionization when a memory cell stores a
logic-1 and does not generate carries through impact ionization
when a memory cell stores a logic-0.
In the embodiment discussed in FIGS. 3A through 3C, bipolar device
30 has been an n-p-n transistor. Persons of ordinary skill in the
art will readily appreciate that by reversing the first and second
connectivity types and inverting the relative values of the applied
voltages memory cell 50 could comprise a bipolar device 30 which is
a p-n-p transistor. Thus the choice of an n-p-n transistor is an
illustrative example for simplicity of explanation in FIGS. 3A
through 3C is not limiting in any way.
FIG. 4A shows an exemplary array 80 of memory cells 50 (four
exemplary instances of memory cell 50 being labeled as 50a, 50b,
50c and 50d) arranged in rows and columns. In many, but not all, of
the figures where exemplary array 80 appears, representative memory
cell 50a will be representative of a "selected" memory cell 50 when
the operation being described has one (or more in some embodiments)
selected memory cells 50. In such figures, representative memory
cell 50b will be representative of an unselected memory cell 50
sharing the same row as selected representative memory cell 50a,
representative memory cell 50c will be representative of an
unselected memory cell 50 sharing the same column as selected
representative memory cell 50a, and representative memory cell 50d
will be representative of a memory cell 50 sharing neither a row or
a column with selected representative memory cell 50a.
Present in FIG. 4A are word lines 70a through 70n, source lines 72a
through 72n, bit lines 74a through 74p, and substrate terminal 78.
Each of the word lines 70a through 70n is associated with a single
row of memory cells 50 and is coupled to the gate 60 of each memory
cell 50 in that row. Similarly, each of the source lines 72a
through 72n is associated with a single row of memory cells 50 and
is coupled to the buried well region 22 of each memory cell 50 in
that row. Each of the bit lines 74a through 74p is associated with
a single column of memory cells 50 and is coupled to the bit line
region 16 of each memory cell 50 in that column. In the holding
operation described in FIGS. 4A through 4C, there is no
individually selected memory cell. Rather cells are selected in
rows by the source lines 72a through 72n and may be selected as
individual rows, as multiple rows, or as all of the rows comprising
array 80.
Substrate 12 is present at all locations under array 80. Persons of
ordinary skill in the art will appreciate that one or more
substrate terminals 78 will be present in one or more locations as
a matter of design choice. Such skilled persons will also
appreciate that while exemplary array 80 is shown as a single
continuous array in FIG. 4A, that many other organizations and
layouts are possible like, for example, word lines may be segmented
or buffered, bit lines may be segmented or buffered, source lines
may be segmented or buffered, the array 80 may be broken into two
or more sub-arrays, control circuits such as word decoders, column
decoders, segmentation devices, sense amplifiers, write amplifiers
may be arrayed around exemplary array 80 or inserted between
sub-arrays of array 80. Thus the exemplary embodiments, features,
design options, etc., described are not limiting in any way.
Turning now to FIG. 4B, array 80 previously discussed is shown
along with multiplexers 40a through 40n and voltage waveforms 42a
through 42n. A periodic pulse of positive voltage can be applied to
the back bias terminals of memory cells 50 through SL terminal 72
as opposed to applying a constant positive bias to reduce the power
consumption of the memory cell 50. FIG. 4B further shows
multiplexers 40a through 40n each coupled to one of the source
lines 72a through 72n that determine the bias voltages applied to
SL terminals 72a through 72n, which will be determined by different
operating modes. The pulsing of the voltage on the SL terminals may
be controlled, for example, by applying pulses of logic signals
like waveforms 42a through 42n to the select input of multiplexers
40a through 40n thereby selecting, for example, ground (0.0 volts)
or a power supply voltage such as Vcc. Many other techniques may be
used to pulse the voltage applied to SL terminals 72a through 72n
like, for example, applying the waveforms 42a through 42n at
different times, or applying them simultaneously, or coupling the
select inputs of multiplexers 42a through 42n together and applying
a single pulsed waveform to all of the multiplexers 42a through 42n
simultaneously (not shown in the figure). Many other options will
readily suggest themselves to persons of ordinary skill in the art.
Thus the described exemplary embodiments are not limiting in any
way.
FIG. 4C shows another method to provide voltage pulses to SL
terminals 72a through 72n of exemplary array 80 of memory cells 50.
The positive input signals to multiplexers 40a through 40n may be
generated by voltage generator circuits 44a through 44n coupled to
one input of each of the multiplexers 40a through 40n.
Alternatively, a single voltage generator circuit may be coupled to
each of the multiplexers 40a through 40n reducing the amount of
overhead circuitry required to refresh the memory cells 50 of array
80. Other embodiments are possible including, for example, applying
the waveforms 42a through 42n at different times, or applying them
simultaneously, or coupling the select inputs of multiplexers 42a
through 42n together and applying a singe pulsed waveform to all of
the multiplexers 42a through 42n simultaneously (not shown in the
figure).
FIG. 4D shows a reference generator circuit suitable for use as
reference generator circuits 44a through 44n in FIG. 4C. The
reference generator includes reference cell 53, which consists of a
modified version of Gated half transistor memory cell 50 described
above with region 25 of the first conductivity type (p-type
conductivity). The p-type 25 region allows for a direct sensing of
the floating body region 24 potential. Region 25 is drawn
separately even though it has the same conductivity type as
floating body region 24 because it may be doped differently to
facilitate contacting it. The reference cell 53 for example can be
configured to be in state logic-1 where the potential of the
floating body region 24 is positive, for example at +0.5V. The
potential sensed through the p-type region is then compared with a
reference value V.sub.REF, e.g. +0.5V, by operational amplifier 27.
If the potential of the floating body region 24 is less than the
reference value, the voltage applied to the back bias terminal 72
(which is connected to buried region 22 of the reference cell 53
and can also be connected to buried region 22 of the Gated half
transistor memory cell 50) is increased by operational amplifier 27
until the potential of the floating body region 24 reaches the
desired reference voltage. If the potential of the floating body 24
region is higher than that of the reference value, the voltage
applied to back bias terminal 72 can be reduced by operational
amplifier 27 until the potential of the floating body region 24
reaches the desired reference voltage. Reference voltage V.sub.REF
may be generated in many different ways like, for example, using a
band gap reference, a resistor string, a digital-to-analog
converter, etc. Similarly alternate voltage generators of types
known in the art may be used
As shown in FIG. 5, the holding/standby operation also results in a
larger memory window by increasing the amount of charge that can be
stored in the floating body 24. Without the holding/standby
operation, the maximum potential that can be stored in the floating
body 24 is limited to the flat band voltage V.sub.FB as the
junction leakage current from floating body 24 to bit line region
16 increases exponentially at floating body potential greater than
V.sub.FB. However, by applying a positive voltage to SL terminal
72, the bipolar action results in a hole current flowing into the
floating body 24, compensating for the junction leakage current
between floating body 24 and bit line region 16. As a result, the
maximum charge V.sub.MC stored in floating body 24 can be increased
by applying a positive bias to the SL terminal 72 as shown in the
graph in FIG. 5. The increase in the maximum charge stored in the
floating body 24 results in a larger memory window.
The holding/standby operation can also be used for multi-bit
operation in memory cell 50. To increase the memory density without
increasing the area occupied by the memory cell, a multi-level
operation is typically used. This is done by dividing the overall
memory window into more than two different levels. In one
embodiment four levels representing two binary bits of data are
used, though many other schemes like, for example, using eight
levels to represent three binary bits of data are possible. In a
floating body memory, the different memory states are represented
by different charge in the floating body 24, as described, for
example, in Tack and Oknonin-2 cited above. However, since the
state with zero charge in the floating body 24 is the most stable
state, the floating body 24 will over time lose its charge until it
reaches the most stable state. In multi-level operation, the
difference of charge representing different states is smaller than
a single-level operation. As a result, a multi-level memory cell is
more sensitive to charge loss.
FIG. 6 shows the floating body 24 net current for different
floating body 24 potential as a function of the voltage applied to
SL terminal 72 with BL, WL and substrate terminals 74, 70, and 78,
grounded. When zero voltage is applied to SL terminal 72, no
bipolar current is flowing into the floating body 24 and as a
result, the stored charge will leak over time. When a positive
voltage is applied to SL terminal 72, hole current will flow into
floating body 24 and balance the junction leakage current to bit
line region 16. The junction leakage current is determined by the
potential difference between the floating body 24 and bit line
region 16, while the bipolar current flowing into floating body 24
is determined by both the SL terminal 72 potential and the floating
body 24 potential. As indicated in FIG. 6, for different floating
body potentials, at a certain SL terminal 72 potential V.sub.HOLD,
the current flowing into floating body 24 is balanced by the
junction leakage between floating body 24 and bit line region 16.
The different floating body 24 potentials represent different
charges used to represent different states of memory cell 50. This
shows that different memory states can be maintained by using the
holding/standby operation described here.
In one embodiment the bias condition for the holding operation for
memory cell 50 is: 0 volts is applied to BL terminal 74, a positive
voltage like, for example, +1.2 volts is applied to SL terminal 72,
0 volts is applied to WL terminal 70, and 0 volts is applied to the
substrate terminal 78. In another embodiment, a negative voltage
may be applied to WL terminal 70. In other embodiments, different
voltages may be applied to the various terminals of memory cell 50
as a matter of design choice and the exemplary voltages described
are not limiting in any way.
The read operation of the memory cell 50 and array 80 of memory
cells will described in conjunction with FIGS. 7 and 8A through 8H.
Any sensing scheme known in the art can be used with memory cell
50. Examples include, for example, the sensing schemes disclosed in
Ohsawa-1 and Ohsawa-2 cited above.
The amount of charge stored in the floating body 24 can be sensed
by monitoring the cell current of the memory cell 50. If memory
cell 50 is in a logic-1 state having holes in the body region 24,
then the memory cell will have a higher cell current (e.g. current
flowing from the BL terminal 74 to SL terminal 72), compared to if
cell 50 is in a logic-0 state having no holes in floating body
region 24. A sensing circuit typically connected to BL terminal 74
can then be used to determine the data state of the memory
cell.
A read operation may be performed by applying the following bias
condition to memory cell 50: a positive voltage is applied to the
selected BL terminal 74, and an even more positive voltage is
applied to the selected WL terminal 70, zero voltage is applied to
the selected SL terminal 72, and zero voltage is applied to the
substrate terminal 78. This has the effect of operating bipolar
device 30 as a backward n-p-n transistor in a manner analogous to
that described for operating bipolar device 30 for a hold operation
as described in conjunction with FIGS. 3A through 3C. The positive
voltage applied to the WL terminal 70 boosts the voltage on the
floating body region 24 by means of capacitive coupling from the
gate 60 to the floating body region 24 through gate insulating
layer 62. This has the effect of increasing the current in bipolar
device 30 when it is on significantly more than it increases the
current when it is off, thus making it easier to sense the data
stored in the memory cell 50. The optimal bias voltage to apply to
WL terminal 70 will vary from embodiment to embodiment and process
to process. The actual voltage applied in any given embodiment is a
matter of design choice.
FIG. 7 shows array 80 of memory cells 50 during a read operation in
one exemplary embodiment of the present invention. Reading a memory
cell 50 in array 80 is more complicated than reading a single cell
as described above, since cells are coupled together along rows by
word lines 70a through 70n and source lines 72a through 72n and
coupled together along columns by bit lines 74a through 74p. In one
exemplary embodiment, about 0.0 volts is applied to the selected SL
terminal 72a, about +0.4 volts is applied to the selected bit line
terminal 74a, about +1.2 volts is applied to the selected word line
terminal 70a, and about 0.0 volts is applied to substrate terminal
78. All the unselected bit line terminals 74b (not shown) through
74p have 0.0 volts applied, the unselected word line terminals 70b
(not shown) through 70n have 0.0 volts applied, and the unselected
SL terminals 72b (not shown) have +1.2 volts applied. FIG. 7 shows
the bias conditions for the selected representative memory cell 50a
and three unselected representative memory cells 50b, 50c, and 50d
in memory array 80, each of which has a unique bias condition.
Persons of ordinary skill in the art will appreciate that other
embodiments of the invention may employ other combinations of
applied bias voltages as a matter of design choice. Such skilled
persons will also realize that the first and second conductivity
types may be reversed and the relative bias voltages may be
inverted in other embodiments.
FIG. 8A also shows the bias condition of the selected
representative memory cell 50a in cross section while FIG. 8B shows
the equivalent circuit diagram illustrating the intrinsic n-p-n
bipolar device 30 under the read bias conditions described
above.
The three cases for unselected memory cells 50 during read
operations are shown in FIGS. 8C, 8E, and 8G, while illustrations
of the equivalent circuit diagrams are shown in FIGS. 8D, 8F, and
8H respectively. The bias conditions for memory cells 50 sharing
the same row (e.g. representative memory cell 50b) and those
sharing the same column (e.g., representative memory cell 50c) as
the selected representative memory cell 50a are shown in FIGS.
8C-8D and FIGS. 8E-8F, respectively, while the bias condition for
memory cells 50 not sharing the same row nor the same column as the
selected representative memory cell 50a (e.g., representative
memory cell 50d) is shown in FIG. 8G-8H.
As shown in FIGS. 8C and 8D, for representative memory cell 50b
sharing the same row as the selected representative memory cell
50a, the SL terminal 72a is now grounded and consequently these
cells will not be at the holding mode. However, because a read
operation is accomplished much faster (on the order of nanoseconds)
compared to the lifetime of the hole charge in the floating body 24
(on the order of milliseconds), it should cause little disruptions
to the charge stored in the floating body.
As shown in FIGS. 8E and 8F, representative memory cell 50c sharing
the same column as the selected memory cell 50a, a positive voltage
is applied to the BL terminal 74a. Less base current will flow into
the floating body 24 due to the smaller potential difference
between SL terminal 72n and BL terminal 74a (i.e. the emitter and
collector terminals of the n-p-n bipolar device 30). However,
because read operation is accomplished much faster (on the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (on the order of milliseconds), it should cause little
disruptions to the charge stored in the floating body.
As shown in FIGS. 8G and 8H, representative memory cell 50d sharing
neither the same row nor the same column as the selected
representative memory cell 50a, the SL terminal 72n will remain
positively charged and the BL terminal 74p will remain grounded. As
can be seen, these cells will be in the holding mode, where memory
cells in the logic-1 state will maintain the charge in floating
body 24 because the intrinsic bipolar device 30 will generate hole
current to replenish the charge in floating body 24 and memory
cells in the logic-0 state will remain in neutral state.
The read operation of the memory cell 50 and array 80 of memory
cells have been described in conjunction with FIGS. 7 through 8H.
Persons of ordinary skill in the art will realize that the drawing
figures are not drawn to scale, that the various voltages described
are illustrative only and will vary from embodiment to embodiment,
that embodiments discussed have been illustrative only, and that
many more embodiments employing the inventive principles of the
invention are possible. For example, the two conductivity types may
be reversed and the relative voltages of the various signals may be
inverted, the memory array 80 may be built as a single array or
broken into sub-arrays, the accompanying control circuits may be
implemented in different ways, different relative or absolute
voltage values may be applied to memory cell 50 or array 80, etc.
Thus the exemplary embodiments, features, bias levels, etc.,
described are not limiting in any way.
A first type of write logic-0 operation of an individual memory
cell 50 is now described with reference to FIGS. 9A and 9B. In FIG.
9A, a negative voltage bias is applied to the back bias terminal
(i.e. SL terminal 72), a zero voltage bias is applied to WL
terminal 70, a zero voltage bias is applied to BL terminal 72 and
substrate terminal 78. Under these conditions, the p-n junction
between floating body 24 and buried well 22 of the selected cell 50
is forward-biased, evacuating any holes from the floating body 24.
In one particular non-limiting embodiment, about -0.5 volts is
applied to source line terminal 72, about 0.0 volts is applied to
word line terminal 70, and about 0.0 volts is applied to bit line
terminal 74 and substrate terminal 78. These voltage levels are
exemplary only may vary from embodiment to embodiment as a matter
of design choice. Thus the exemplary embodiments, features, bias
levels, etc., described are not limiting in any way.
In FIG. 9B, an alternative embodiment of memory cell 50 is shown
where substrate 12 is replaced by region 12A of the first
conductivity type (p-type in the figure) which is a well inside
substrate 29 of the second conductivity type (n-type in the
figure). This arrangement overcomes an undesirable side effect of
the embodiment of FIG. 9A where lowering the buried well region 22
voltage on buried well terminal 72 to approximately -0.5V to
forward bias the p-n junction between buried well 22 and floating
body 24 also forward biases the p-n junction between buried well 22
and substrate 12 resulting in unwanted substrate current. The
embodiment of FIG. 9B allows the well 12A to be lowered by applying
the same voltage to well terminal 78 as buried layer terminal 72
thus preventing the p-n diode between those regions to forward
bias. The substrate 29 is preferably biased to 0.0V through
substrate terminal 31 as shown in FIG. 9B. These voltage levels are
exemplary only may vary from embodiment to embodiment as a matter
of design choice, Thus the exemplary embodiments, features, bias
levels, etc., described are not limiting in any way.
FIG. 10 shows an example of bias conditions for the selected and
unselected memory cells 50 during the first type of write logic-0
operation (as described in FIG. 9A) in memory array 80. For the
selected representative memory cells 50a and 50b, the negative bias
applied to SL terminal 72a causes large potential difference
between floating body 24 and buried well region 22. Because the
buried well 22 is shared among multiple memory cells 50, logic-0
will be written into all memory cells 50 including memory cells 50a
and 50b sharing the same SL terminal 72a simultaneously.
FIGS. 11A through 11B illustrate an example of bias conditions and
an equivalent circuit diagram illustrating the intrinsic n-p-n
bipolar devices 30 of unselected memory cells 50 like
representative memory cells 50c and 50d in array 80 during the
first type of logic-0 write operations. In particular
representative memory cell 50d will be discussed for clarity of
presentation though the principles apply to all unselected memory
cells 50. Since the logic-0 write operation only involves a
negative voltage to the selected SL terminal 72a, the memory cells
50 coupled to the unselected SL terminals 72b (not shown in FIG.
10) through 72n are placed in a holding operation by placing a
positive bias condition on SL terminals 72b through 72n. As can be
seen in FIGS. 11A and 11B, the unselected memory cells will be in a
holding operation, with the BL terminal at about 0.0 volts, WL
terminal at zero voltage, and the unselected SL terminal positively
biased.
As shown in FIG. 12, a second type of write logic-0 operation can
also be performed by applying a negative bias to the BL terminal 74
as opposed to the SL terminal 72. In FIG. 12, the selected memory
cells 50 include representative memory cells 50a and 50c and all
the memory cells 50 that share the selected bit line 74a. The SL
terminal 72 will be positively biased, while zero voltage is
applied to the substrate terminal 78, and zero voltage is applied
to the WL terminal 70. Under these conditions, all memory cells
sharing the same BL terminal 74 will be written to the logic-0
state.
The first and second types of write logic-0 operations referred to
above each has a drawback that all memory cells 50 sharing either
the same SL terminal 72 (the first type--row write logic-0) or the
same BL terminal 74 will (the second type--column write logic-0) be
written to simultaneously and as a result, does not allow writing
logic-0 to individual memory cells 50. To write arbitrary binary
data to different memory cells 50, a write logic-0 operation is
first performed on all the memory cells to be written followed by
one or more write logic-1 operations on the bits that must be
written to logic-1.
A third type of write logic-0 operation that allows for individual
bit writing can be performed on memory cell 50 by applying a
positive voltage to WL terminal 70, a negative voltage to BL
terminal 74, zero or positive voltage to SL terminal 72, and zero
voltage to substrate terminal 78. Under these conditions, the
floating body 24 potential will increase through capacitive
coupling from the positive voltage applied to the WL terminal 70.
As a result of the floating body 24 potential increase and the
negative voltage applied to the BL terminal 74, the p-n junction
between 24 and bit line region 16 is forward-biased, evacuating any
holes from the floating body 24.
To reduce undesired write logic-0 disturb to other memory cells 50
in the memory array 80, the applied potential can be optimized as
follows: if the floating body 24 potential of state logic-1 is
referred to as V.sub.FB1, then the voltage applied to the WL
terminal 70 is configured to increase the floating body 24
potential by V.sub.FB1/2 while -V.sub.FB1/2 is applied to BL
terminal 74. Additionally, either ground or a slightly positive
voltage may also be applied to the BL terminals 74 of unselected
memory cells 50 that do not share the same BL terminal 74 as the
selected memory cell 50, while a negative voltage may also be
applied to the WL terminals 70 of unselected memory cells 50 that
do not share the same WL terminal 70 as the selected memory cell
50.
As illustrated in FIG. 13, the following bias conditions are
applied to the selected representative memory cell 50a in exemplary
memory array 80 to perform an individual write logic-0 operation
exclusively in representative memory cell 50a: a potential of about
0.0 volts to SL terminal 72a, a potential of about -0.2 volts to BL
terminal 74a, a potential of about +0.5 volts is applied to word
line terminal 70a, and about 0.0 volts is applied to substrate
terminal 78. In the rest of array 80 about +1.2 volts is applied to
unselected SL terminals 72 (including SL terminal 72n), about 0.0
volts (or possibly a slightly positive voltage) is applied to
unselected BL terminals 74 (including BL terminal 74p), and about
0.0 volts is applied to unselected WL terminal 70 (including WL
terminal 70n). Persons of ordinary skill in the art will appreciate
that the voltage levels in FIG. 13 are illustrative only and that
different embodiments will have different voltage levels as a
matter of design choice.
The bias conditions shown in FIG. 13 of the selected representative
memory cell 50a in memory array 80 to perform the individual bit
write logic-0 operation are further illustrated in FIGS. 14A and
14B. As discussed above, the potential difference between floating
body 24 and bit line region 16 connected to BL terminal 74a is now
increased due to the capacitive coupling from raising WL terminal
70a from ground to +0.5V, resulting in a higher forward bias
current than the base hole current generated by the n-p-n bipolar
device 30 formed by buried well region 22 connected to SL terminal
72a, floating body 24, and bit line region 16. The result is that
holes will be evacuated from floating body 24.
The unselected memory cells 50 in memory array 80 under the bias
conditions of FIG. 13 during the individual bit write logic-0
operation are shown in FIGS. 14C through 14H. The bias conditions
for memory cells sharing the same row (e.g. representative memory
cell 50b) as the selected representative memory cell 50a are
illustrated in FIGS. 14C and 14D, and the bias conditions for
memory cells sharing the same column (e.g. representative memory
cell 50c) as the selected representative memory cell 50a are shown
in FIGS. 14E and 14F, and the bias conditions for memory cells
sharing neither the same row nor the same column (e.g.
representative memory cell 50d) as the selected representative
memory cell 50a are shown in FIGS. 14G and 14H.
As shown in FIGS. 14C and 14D, the floating body 24 potential of
memory cell 50b sharing the same row as the selected representative
memory cell 50a will increase due to capacitive coupling from WL
terminal 70 by .DELTA.V.sub.FB. For memory cells in state logic-0,
the increase in the floating body 24 potential is not sustainable
as the forward bias current of the p-n diodes formed by floating
body 24 and junction 16 will evacuate holes from floating body 24.
As a result, the floating body 24 potential will return to the
initial state logic-0 equilibrium potential. For memory cells in
state logic-1, the floating body 24 potential will initially also
increase by .DELTA.V.sub.FB, which will result in holes being
evacuated from floating body 24. After the positive bias on the WL
terminal 70 is removed, the floating body 24 potential will
decrease by .DELTA.V.sub.FB. If the initial floating body 24
potential of state logic-1 is referred to as V.sub.FB1, the
floating body 24 potential after the write logic-0 operation will
become V.sub.FB1-.DELTA.V.sub.FB. Therefore, the WL potential needs
to be optimized such that the decrease in floating body potential
of memory cells 50 in state logic-1 is not too large during the
time when the positive voltage is applied to (and subsequently
removed from) WL terminal 70a. For example, the maximum floating
body potential increase due to the coupling from the WL potential
cannot exceed V.sub.FB1/2. Thus in some embodiments it may be
advantageous to have a slightly positive voltage on unselected BL
terminal 74p. This means that bipolar device 30 can only evacuate
holes in reverse operation (e.g., only the p-n junction between the
floating body 24 and buried well 22 will be on enough to evacuate
holes from the floating body region 24) which may minimize the
reduction of holes in floating body region 24 in the logic-1
state.
As shown in FIGS. 14E and 14F, for representative memory cell 50c
sharing the same column as the selected representative memory cell
50a, a negative voltage is applied to the BL terminal 74a,
resulting in an increase in the potential difference between
floating body 24 and bit line region 16 connected to the BL
terminal 74a. As a result, the p-n diode formed between floating
body 24 and bit line region 16 will be forward biased. For memory
cells in the logic-0 state, the increase in the floating body 24
potential will not change the initial state from logic-0 as there
is initially no hole stored in the floating body 24. For memory
cells in the logic-1 state, the net effect is that the floating
body 24 potential after write logic-0 operation will be reduced.
Therefore, the BL potential also needs to be optimized such that
the decrease in floating body potential of memory cells 50 in state
logic-1 is not too large during the time when the negative voltage
is applied to BL terminal 74a. For example, the -V.sub.FB1/2 is
applied to the BL terminal 74a.
As shown in FIGS. 14G and 14H, memory cell 50d sharing neither the
same row nor the same column as the selected representative memory
cell 50a, these cells will be in a holding mode as positive voltage
is applied to the SL terminal 72n, zero voltage is applied to the
BL terminal 74p, and zero or negative voltage is applied to WL
terminal 70n, and zero voltage is applied to substrate terminal
78.
Three different methods for performing a write logic-0 operation on
memory cell 50 have been disclosed. Many other embodiments and
component organizations are possible like, for example, reversing
the first and second conductivity types while inverting the
relative voltage biases applied. An exemplary array 80 has been
used for illustrative purposes, but many other possibilities are
possible like, for example, applying different bias voltages to the
various array line terminals, employing multiple arrays, performing
multiple single bit write logic-0 operations to multiple selected
bits in one or more arrays or by use of decoding circuits,
interdigitating bits so as to conveniently write logic-0s to a data
word followed by writing logic-1s to selected ones of those bits,
etc. Such embodiments will readily suggest themselves to persons of
ordinary skill in the art familiar with the teachings and
illustrations herein. Thus the exemplary embodiments, features,
bias levels, etc., described are not limiting in any way.
A write logic-1 operation may be performed on memory cell 50
through impact ionization as described, for example, with reference
to Lin cited above, or through a band-to-band tunneling mechanism
(also known as Gate Induced Drain Leakage or GIDL), as described,
for example with reference to Yoshida cited above. An example of a
write logic-1 operation using the GIDL method is described in
conjunction with FIGS. 15 and 15A through 15H while an example of a
write logic-1 operation using the impact ionization method is
described in conjunction with FIGS. 16 and 16A through 16H.
In FIG. 15 an example of the bias conditions of the array 80
including selected representative memory cell 50a during a
band-to-band tunneling write logic-1 operation is shown. The
negative bias applied to the WL terminal 70a and the positive bias
applied to the BL terminal 74a results in hole injection to the
floating body 24 of the selected representative memory cell 50a.
The SL terminal 72a and the substrate terminal 78 are grounded
during the write logic-1 operation.
The negative voltage on WL terminal 70 couples the voltage
potential of the floating body region 24 in representative memory
cell 50a downward. This combined with the positive voltage on BL
terminal 74a creates a strong electric field between the bit line
region 16 and the floating body region 24 in the proximity of gate
60 (hence the "gate induced" portion of GIDL) in selected
representative memory cell 50a. This bends the energy bands sharply
upward near the gate and drain junction overlap region, causing
electrons to tunnel from the valence band to the conduction band,
leaving holes in the valence band. The electrons which tunnel
across the energy band become the drain leakage current (hence the
"drain leakage" portion of GIDL), while the holes are injected into
floating body region 24 and become the hole charge that creates the
logic-1 state. This process is well known in the art and is
illustrated in Yoshida (specifically FIGS. 2 and 6 on page 3 and
FIG. 9 on page 4) cited above.
As shown in FIGS. 15A through 15B, the following bias conditions
may be applied to the selected representative memory cell 50a: a
potential of about 0.0 volts is applied to SL terminal 72a, a
potential of about +1.2 volts is applied to BL terminal 74a, a
potential of about -1.2 volts is applied to WL terminal 70a, and
about 0.0 volts is applied to substrate terminal 78.
Elsewhere in array 80 the following bias conditions are applied to
the terminals for unselected memory cells 50 including
representative memory cells 50b, 50c and 50d: about +1.2 volts is
applied to SL terminal 72n, about 0.0 volts is applied to BL
terminal 74p, a potential of about 0.0 volts is applied to WL
terminal 70n, and about 0.0 volts is applied to substrate terminal
78. FIG. 15 shows the bias condition for the selected and
unselected memory cells in memory array 80. However, these voltage
levels may vary from embodiment to embodiment of the present
invention and are exemplary only and are in no way limiting.
The unselected memory cells during write logic-1 operations are
shown in FIGS. 15C through 15H. The bias conditions for memory
cells sharing the same row (e.g. representative memory cell 50b)
are shown in FIGS. 15C and 15D. The bias conditions for memory
cells sharing the same column as the selected representative memory
cell 50a (e.g. representative memory cell 50c) are shown in FIGS.
15E and 15F. The bias conditions for memory cells 50 not sharing
the same row nor the same column as the selected representative
memory cell 50a (e.g. representative memory cell 50d) are shown in
FIGS. 15G and 15H.
As illustrated in FIGS. 15C and 15D, representative memory cell
50b, sharing the same row as the selected representative memory
cell 50a, has both terminals 72a and 74p grounded, while about -1.2
volts is applied to WL terminal 70a. Because SL terminal 70a is
grounded, memory cell 50b will not be at the holding mode since
there is no voltage across between the emitter and collector
terminals of the n-p-n bipolar device 30 turning it off. However,
because the write logic-1 operation is accomplished much faster (on
the order of nanoseconds) compared to the lifetime of the charge in
the floating body 24 (on the order of milliseconds), it should
cause little disruption to the charge stored in the floating
body.
As illustrated in FIGS. 15E and 15F, for representative memory cell
50c sharing the same column as the selected memory cell, a positive
voltage is applied to the BL terminal 74n. No base current will
flow into the floating body 24 because there is no potential
difference between SL terminal 72n and BL terminal 74a (i.e. there
is no voltage between the emitter and collector terminals of the
n-p-n bipolar device 30 turning it off). However, because a write
operation is accomplished much faster (in the order of nanoseconds)
compared to the lifetime of the charge in the floating body 24 (in
the order of milliseconds), it should cause little disruption to
the charge stored in the floating body.
As illustrated in FIGS. 15G and 15H, for memory cell 50d sharing
neither the same row nor the same column as the selected memory
cell, the SL terminal 72n will remain positively charged while the
gate terminal 70n and the BL terminal 74p remain grounded. As can
be seen, these cells will be at holding mode. Memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30a will generate holes current to
replenish the charge in floating body 24, while memory cells in
state logic-0 will remain in neutral state.
FIG. 16 shows a write logic-1 operation using the impact ionization
method. In this case, both the gate 60 and bit line 16 of the
memory cell 50 to be written are biased at a positive voltage. This
is similar to the holding operation described earlier in
conjunction with FIGS. 3A through 4D which also uses impact
ionization to supply hole current to the floating body 24. However
in the holding operation, the n-p-n bipolar device 30 stays off
when a logic-0 is stored in memory cell 50 and impact ionization
current only flows when a logic-1 is stored in the cell restoring
the charge level in the floating body 24 to a full logic-1 level.
By contrast, in the case of a write logic-1 operation using impact
ionization, the voltage on the gate terminal is positive rather
than zero. The action of raising the gate 60 to a positive voltage
has the effect of raising the voltage potential of the floating
body region 24 due to capacitive coupling across the gate
insulating layer 62 which causes the n-p-n bipolar transistor 30 to
turn on regardless of whether or not a logic-1 or logic-0 is stored
in memory cell 50. This causes impact ionization current to flow
charging the floating body 24 to the logic-1 state regardless of
the data originally stored in the cell.
In the exemplary embodiment shown in FIG. 16, the selected word
line terminal 70a is biased at about +1.2V while the unselected
word line terminals 70b (not shown) through 70n are biased at about
0.0V, the selected bit line terminal 74a is also biased at about
+1.2V while the unselected bit line terminals 74b through 74p are
biased at about 0.0V, the selected source line 72a is biased at
about 0.0V, while the unselected source line terminals 72b (not
shown) through 72n are biased at about +1.2V, and the substrate
terminal 78 is biased at about 0.0V. These voltage bias levels are
exemplary only and will vary from embodiment to embodiment and are
thus in no way limiting.
As shown in FIGS. 16A through 16B, selected representative memory
cell 50a is shown with gate 60 coupled to WL terminal 70A biased at
+1.2V, bit line region 16 coupled to BL terminal 74a biased at
+1.2V, and buried layer 22 coupled to source line terminal 72a
biased at 0.0V. In this state, impact ionization current flows into
the cell from BL terminal 74a injecting holes into the floating
body region 24 writing a logic-1 state into representative memory
cell 50a.
As shown in FIGS. 16C through 16D, unselected representative memory
cell 50b, sharing a row but not a column with selected
representative memory cell 50a, is shown with gate 60 coupled to WL
terminal 70a biased at +1.2V, bit line region 16 coupled to BL
terminal 74p biased at 0.0V, and buried layer 22 coupled to source
line terminal 72a biased at 0.0V. In this state, the
collector-to-emitter voltage of n-p-n bipolar device 30 is 0.0V
causing the device to be off protecting the contents of
representative memory cell 50b.
As shown in FIGS. 16E through 16F, unselected representative memory
cell 50c, sharing a column but not a row with selected
representative memory cell 50a, is shown with gate 60 coupled to WL
terminal 70n biased at 0.0V, bit line region 16 coupled to BL
terminal 74a biased at +1.2V, and buried layer 22 coupled to source
line terminal 72n biased at +1.2V. In this state, the n-p-n bipolar
device 30 will be off since there is no voltage difference between
the collector and emitter terminals of n-p-n bipolar device 30.
As shown in FIGS. 16G through 16H, unselected representative memory
cell 50d, sharing neither a row nor a column with selected
representative memory cell 50a, is shown with gate 60 coupled to WL
terminal 70n biased at 0.0V, bit line region 16 coupled to BL
terminal 74p biased at 0.0V, and buried layer 22 coupled to source
line terminal 72n biased at +1.2V. As can be seen, these cells will
be at holding mode. Memory cells in state logic-1 will maintain the
charge in floating body 24 because the intrinsic bipolar device 30a
will generate holes current to replenish the charge in floating
body 24, while memory cells in state logic-0 will remain in neutral
state.
FIG. 17A shows a top view of an embodiment of a partial memory
array including Gated half transistor memory cell 450 according to
the present invention and FIG. 17B shows memory cell 450 in
isolation. FIGS. 17C and 17D show the memory cell 450 cross
sections along the I-I' line and II-II' cut lines, respectively,
while FIG. 17E shows a method of contacting the buried well and
substrate layers beneath the cells. FIGS. 18A through 18H show
memory array 480 comprised of rows and columns of memory cell 450.
The primary difference between memory cell 50 and memory cell 450
is that while insulating layers 26 isolate the buried layer 22
between memory cells in adjacent rows in memory cell 50, in memory
cell 450 the regions occupied by insulating layer 26 are replaced
by insulating layer 28. Thus memory cell 450 is surrounded by
insulating layer 28 on all four sides and the buried layer 22 is
continuously connected as a single "source line" amongst all of the
memory cells 450 in memory array 480. This makes for a memory array
that is very similar to memory array 80, however some operations
will be different as described below in conjunction with FIGS. 18A
through 18F. As was the case with memory cell 50 in memory cell 80,
there is no contact to the buried layer 22 within the boundary of
memory cell 450.
Referring to FIGS. 17C and 17D together, the cell 450 includes a
substrate 12 of a first conductivity type such as a p-type, for
example. Substrate 12 is typically made of silicon, but may also
comprise, for example, germanium, silicon germanium, gallium
arsenide, carbon nanotubes, or other semiconductor materials. In
some embodiments of the invention, substrate 12 can be the bulk
material of the semiconductor wafer. In other embodiments,
substrate 12 can be a well of the first conductivity type embedded
in either a well of the second conductivity type or, alternatively,
in the bulk of the semiconductor wafer of the second conductivity
type, such as n-type, for example, (not shown in the figures) as a
matter of design choice. To simplify the description, the substrate
12 will is drawn as the semiconductor bulk material as it is in
FIGS. 17C and 17D though it may also be a well in a substrate of
material of the second type of conductivity.
A buried layer 22 of a second conductivity type such as n-type, for
example, is provided in the substrate 12. Buried layer 22 may be
formed by an ion implantation process on the material of substrate
12. Alternatively, buried layer 22 can also be grown epitaxially on
top of substrate 12.
A floating body region 24 of the first conductivity type, such as
p-type, for example, is bounded on top by bit line region 16 an
insulating layer 62, on the sides by insulating layer 28, and on
the bottom by buried layer 22. Floating body 24 may be the portion
of the original substrate 12 above buried layer 22 if buried layer
22 is implanted. Alternatively, floating body 24 may be epitaxially
grown. Depending on how buried layer 22 and floating body 24 are
formed, floating body 24 may have the same doping as substrate 12
in some embodiments or a different doping, if desired in other
embodiments, as a matter of design choice.
Insulating layers 28 (like, for example, shallow trench isolation
(STI)), may be made of silicon oxide, for example, though other
insulating materials may be used. Insulating layers 28 insulate
cell 450 from neighboring cells 450 when multiple cells 450 are
joined in an array 480 to make a memory device as illustrated in
FIGS. 18A-18F. Insulating layer 28 insulates neighboring body
regions 24, but not the buried layer 22, allowing the buried layer
22 to be continuous (i.e. electrically conductive) under the entire
array 480.
A bit line region 16 having a second conductivity type, such as
n-type, for example, is provided in floating body region 24 and is
exposed at surface 14. Bit line region 16 is formed by an
implantation process formed on the material making up substrate 12,
according to any implantation process known and typically used in
the art. Alternatively, a solid state diffusion process could be
used to form bit line region 16.
A gate 60 is positioned in between the bit line region 16 and
insulating layer 28 and above the floating body region 24. The gate
60 is insulated from floating body region 24 by an insulating layer
62. Insulating layer 62 may be made of silicon oxide and/or other
dielectric materials, including high-K dielectric materials, such
as, but not limited to, tantalum peroxide, titanium oxide,
zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 60
may be made of, for example, polysilicon material or metal gate
electrode, such as tungsten, tantalum, titanium and their
nitrides.
Memory cell 450 further includes word line (WL) terminal 70
electrically connected to gate 60, bit line (BL) terminal 74
electrically connected to bit line region 16, source line (SL)
terminal 72 electrically connected to buried layer 22, and
substrate terminal 78 electrically connected to substrate 12.
As shown in FIG. 17E, contact between SL terminal 72 and buried
layer 22 can be made through region 20 having a second conductivity
type, and which is electrically connected to buried well region 22,
while contact between substrate terminal 78 and substrate region 12
can be made through region 21 having a first conductivity type, and
which is electrically connected to substrate region 12.
The SL terminal 72 connected to the buried layer region 22 serves
as a back bias terminal, i.e. a terminal at the back side of a
semiconductor transistor device, usually at the opposite side of
the gate of the transistor.
Comparing the structure of the memory device 450, for example, as
shown in FIG. 17C to the structure of transistor devices 100, 100A
and 100B in FIGS. 52A through 52C, it can be seen that the memory
device of present invention constitutes a smaller structure
relative to the MOSFET devices 100, 100A and 100B, where only one
region of a second conductivity type is present at the surface of
the silicon substrate. Thus, memory cell 450 of the present
invention provides an advantage that it consists of only one region
of second conductivity at the surface (i.e. bit line region 16 as
opposed to regions 84 and 86 or regions 84A and 86A) and hence
requires only one contact per memory cell 50 (i.e. to create a
connection between bit line region 16 and terminal 74).
Persons of ordinary skill in the art will appreciate that in FIGS.
17A through 17E and that the first and second conductivity types
can be reversed in memory cell 50 as a matter of design choice and
that the labeling of regions of the first conductivity type as
p-type and the second conductivity type as p-type is illustrative
only and not limiting in any way. Thus the first and second
conductivity types can be p-type and n-type respectively in some
embodiments of memory cell 50 and be n-type and p-type respectively
in other embodiments. Further, such skilled persons will realize
that the relative doping levels of the various regions of either
conductivity type will also vary as a matter of design choice, and
that there is no significance to the absence of notation signifying
higher or lower doping levels such as p+ or p- or n+ or n- in any
of the diagrams.
FIG. 18A shows an exemplary memory array 480 of memory cells 450
(four exemplary instances of memory cell 450 being labeled as 450a,
450b, 450c and 450d) arranged in rows and columns. In many, but not
all, of the figures where exemplary memory array 480 appears,
representative memory cell 450a will be representative of a
"selected" memory cell 450 when the operation being described has
one (or more in some embodiments) selected memory cells 450. In
such figures, representative memory cell 450b will be
representative of an unselected memory cell 450 sharing the same
row as selected representative memory cell 450a, representative
memory cell 450c will be representative of an unselected memory
cell 450 sharing the same column as selected representative memory
cell 450a, and representative memory cell 450d will be
representative of a memory cell 450 sharing neither a row or a
column with selected representative memory cell 450a.
Present in FIG. 18A are word lines 70a through 70n, source line
terminal 72X, bit lines 74a through 74p, and substrate terminal 78.
Each of the word lines 70a through 70n is associated with a single
row of memory cells 450 and is coupled to the gate 60 of each
memory cell 450 in that row. Each of the bit lines 74a through 74p
is associated with a single column of memory cells 450 and is
coupled to the bit line region 16 of each memory cell 450 in that
column. It is noteworthy that while the source line terminal 72X is
really no longer a control line terminal associated with the source
line 72 of a row of memory cells 450 but a control terminal
associated with all of the memory cells 450 in exemplary memory
array 480, it will still be referred to as "source line" terminal
72X to minimize confusion since it still serves that function for
each individual memory cell 450.
Substrate 12 and buried layer 22 are both present at all locations
under array 480. Persons of ordinary skill in the art will
appreciate that one or more substrate terminals 78 and one or more
buried well terminals 72 will be present in one or more locations
as a matter of design choice. Such skilled persons will also
appreciate that while exemplary array 480 is shown as a single
continuous array in FIG. 18A, that many other organizations and
layouts are possible like, for example, word lines may be segmented
or buffered, bit lines may be segmented or buffered, source lines
may be segmented or buffered, the array 480 may be broken into two
or more sub-arrays, control circuits such as word decoders, column
decoders, segmentation devices, sense amplifiers, write amplifiers
may be arrayed around exemplary array 480 or inserted between
sub-arrays of array 480. Thus the exemplary embodiments, features,
design options, etc., described are not limiting in any way.
FIG. 18B illustrates an array hold operation on exemplary memory
array 480. For all memory cells 450 in the array 480, the hold
operation is performed simultaneously by applying about +1.2V to
the source line terminal 72 while applying about 0.0V to the word
line terminals 70a through 70n, the bit line terminals 74a through
74p, and the substrate terminal 78. This bias condition causes each
of the memory cells 450 in the array 480 storing a logic-1 to have
its intrinsic bipolar transistor 30 turned on to restore the hole
charge on its floating body 24 as discussed above. Simultaneously,
this bias condition causes each of the memory cells 450 in the
array 480 storing a logic-0 to have its intrinsic bipolar
transistor 30 turned off to retain charge neutrality in its
floating body 24 as previously discussed. The voltages applied are
exemplary only, may vary from embodiment to embodiment and are in
no way limiting.
FIG. 18C illustrates a single cell read operation of selected
representative memory cell 450a in exemplary memory array 450. To
accomplish this, the selected word line terminal 70a is biased to
approximately +1.2V while the unselected word line terminals 70b
(not shown) through 70n are biased to about 0.0V, the selected bit
line terminal 74a is biased to approximately +0.4V while the
unselected bit line terminals 74b through 74p are biased to about
0.0V, the source line terminal 72 is biased to about 0.0V, and the
substrate terminal is biased to about 0.0V. The voltages applied
are exemplary only, may vary from embodiment to embodiment, and are
in no way limiting.
This has the effect of operating bipolar device 30 as a backward
n-p-n transistor in a manner analogous to that described for
operating bipolar device 30 for a hold operation as described in
conjunction with FIGS. 3A through 3C.
The capacitive coupling between the word line terminal 70a and the
floating body 24 of selected memory cell 450a increase the
differentiation in the read current between the logic-1 and logic-0
states as previously described. The optimal bias voltage to apply
to WL terminal 70 will vary from embodiment to embodiment and
process to process. The actual voltage applied in any given
embodiment is a matter of design choice.
Unselected representative memory cell 450b, which shares a row with
selected representative memory cell 450a, has its bipolar device 30
turned off because there is no voltage between the collector and
emitter terminals. It retains its logic state during the short
duration of the read operation.
Unselected representative memory cell 450c, which shares a column
with selected representative memory cell 450a, will either be off
or be in a weak version of the holding operation depending on the
device characteristics of the process of any particular embodiment.
It also retains its logic state during the short duration of the
read operation.
Unselected representative memory cell 450d, which shares neither a
row nor a column with selected representative memory cell 450a, has
its bipolar device 30 turned off because there is no voltage
between the collector and emitter terminals. It too retains its
logic state during the short duration of the read operation.
FIG. 18D illustrates an array write logic-0 operation of all the
memory cells 450 in exemplary memory array 450. To accomplish this,
all the word line terminals 70a through 70n are biased to
approximately 0.0V, all the bit line terminals 74a through 74p are
biased to approximately -1.2V, the source line terminal 72 is
biased to about 0.0V, and the substrate terminal is biased to about
0.0V. The voltages applied are exemplary only, may vary from
embodiment to embodiment, and are in no way limiting.
This bias condition forward biases the p-n junction between the
floating body 24 and the bit line region 16 turning on the
intrinsic bipolar device 30 in each of the memory cells 450 as
previously described. This evacuates all of the holes in the
floating body regions 24 writing a logic-0 to all of the memory
cells 450 in array 480.
FIG. 18E illustrates a column write logic-0 operation of one column
of the memory cells 450 in exemplary memory array 450. To
accomplish this, all the word line terminals 70a through 70n are
biased to approximately 0.0V, selected the bit line terminal 74a is
biased to approximately -1.2V while the unselected bit line
terminals 74b through 74p are biased to about 0.0V, the source line
terminal 72 is biased to about +1.2V, and the substrate terminal is
biased to about 0.0V. The voltages applied are exemplary only, may
vary from embodiment to embodiment, and are in no way limiting.
This bias condition forward biases the p-n junction between the
floating body 24 and the bit line region 16 turning on the
intrinsic bipolar device 30 in each of the memory cells 450 coupled
to bit line 74a, including representative memory cells 450a and
450c, as previously described. This evacuates all of the holes in
the floating body regions 24 writing a logic-0 to all of the memory
cells 450 in the selected column.
The remaining memory cells 450 in array 480, including
representative memory cells 450b and 450d, are in a holding
operation and will retain their logic state during the write
logic-0 operation.
FIG. 18F illustrates a single cell write logic-0 operation of
selected representative memory cell 450a in exemplary memory array
450. To accomplish this, the selected word line terminal 70a is
biased to approximately +0.5V while the unselected word line
terminals 70b (not shown) through 70n are biased to about -1.2V,
the selected bit line terminal 74a is biased to approximately -0.2V
while the unselected bit line terminals 74b through 74p are biased
to about 0.0V, the source line terminal 72 is biased to about 0.0V,
and the substrate terminal is biased to about 0.0V. The voltages
applied are exemplary only, may vary from embodiment to embodiment,
and are in no way limiting.
This bias condition forward biases the p-n junction between the
floating body 24 and the bit line region 16 turning on the
intrinsic bipolar device 30 in selected representative memory cell
450a. The capacitive coupling between the word line terminal 70a
and the floating body 24 of selected memory cell 450a causes
bipolar device 30 to turn on evacuating the holes in floating body
region 24 as previously described.
Unselected representative memory cell 450b, which shares a row with
selected representative memory cell 450a, has its bipolar device 30
turned off because there is no voltage between the collector and
emitter terminals. It retains its logic state during the short
duration of the read operation.
Unselected representative memory cell 450c, which shares a column
with selected representative memory cell 450a, has the voltage
potential of its floating body temporarily lowered because the
negative capacitive coupling between its floating body 24 its gate
60 (coupled to word line terminal 70n) preventing its bipolar
device 30 from turning on. It also retains its logic state during
the short duration of the read operation, and the voltage potential
of its floating body 24 is restored to its previous level by the
positive coupling between its floating body 24 its gate 60 (coupled
to word line terminal 70n) when the word line terminal is returned
to its nominal value of about 0.0V after the operation is
complete.
Unselected representative memory cell 450d, which shares neither a
row nor a column with selected representative memory cell 450a, has
its bipolar device 30 turned off because there is no voltage
between the collector and emitter terminals. It too retains its
logic state during the short duration of the read operation.
FIG. 18G illustrates a single cell write logic-1 operation using a
GIDL mechanism in selected representative memory cell 450a in
exemplary memory array 450. To accomplish this, the selected word
line terminal 70a is biased to approximately -1.2V while the
unselected word line terminals 70b (not shown) through 70n are
biased to about 0.0V, the selected bit line terminal 74a is biased
to approximately +1.2V while the unselected bit line terminals 74b
through 74p are biased to about 0.0V, the source line terminal 72
is biased to about 0.0V, and the substrate terminal is biased to
about 0.0V. The voltages applied are exemplary only, may vary from
embodiment to embodiment, and are in no way limiting.
This bias condition causes selected representative memory cell 450a
to conduct current due to the GIDL mechanism discussed with
reference to Yoshida cited above. The combination of -1.2V on word
line terminal and +1.2V on bit line terminal 74a creates the strong
electric field necessary to produce GIDL current from bit line 74a
into representative memory cell 450a generating sufficient hole
charge in its floating body 24 to place it in the logic-1
state.
Unselected representative memory cell 450b, which shares a row with
selected representative memory cell 450a, has its bipolar device 30
turned off because there is no voltage between the collector and
emitter terminals. It retains its logic state during the short
duration of the read operation.
Unselected representative memory cell 450c, which shares a column
with selected representative memory cell 450a, is in the holding
state. It also retains its logic state during the short duration of
the write logic-1 operation.
Unselected representative memory cell 450d, which shares neither a
row nor a column with selected representative memory cell 450a, has
its bipolar device 30 turned off because there is no voltage
between the collector and emitter terminals. It too retains its
logic state during the short duration of the read operation.
FIG. 18H illustrates a single cell write logic-1 operation using an
impact ionization mechanism in selected representative memory cell
450a in exemplary memory array 450. To accomplish this, the
selected word line terminal 70a is biased to approximately +1.2V
while the unselected word line terminals 70b (not shown) through
70n are biased to about 0.0V, the selected bit line terminal 74a is
biased to approximately +1.2V while the unselected bit line
terminals 74b through 74p are biased to about 0.0V, the source line
terminal 72 is biased to about 0.0V, and the substrate terminal is
biased to about 0.0V. The voltages applied are exemplary only, may
vary from embodiment to embodiment, and are in no way limiting.
This bias condition causes selected representative memory cell 450a
to conduct current due to the impact ionization mechanism discussed
with reference to Lin cited above. The combination of +1.2V on word
line terminal and +1.2V on bit line terminal 74a turns on the
bipolar device 30 in representative memory cell 450a regardless of
its prior logic state and generating sufficient hole charge in its
floating body 24 to place it in the logic-1 state.
Unselected representative memory cell 450b, which shares a row with
selected representative memory cell 450a, has its bipolar device 30
turned off because there is no voltage between the collector and
emitter terminals. It retains its logic state during the short
duration of the read operation.
Unselected representative memory cell 450c, which shares a column
with selected representative memory cell 450a, is in the holding
state. It also retains its logic state during the short duration of
the write logic-1 operation.
Unselected representative memory cell 450d, which shares neither a
row nor a column with selected representative memory cell 450a, has
its bipolar device 30 turned off because there is no voltage
between the collector and emitter terminals. It too retains its
logic state during the short duration of the read operation.
In the previous embodiments, a single binary bit has been written
to, read from, and maintained in a single memory cell 50 or 450.
While this approach makes for the simplest support circuitry, the
simplest operating methods, and the largest noise margins, greater
memory density may be achieved by storing two or more bits per
memory cell 50 or 450 at the cost of increasing the complexity of
the support circuitry and operating methods. Additionally, the
noise margin is also reduced because the voltage window inside
memory cell 50 or 450 is shared by more than two logic levels.
Preferably the information stored in memory cell 50 or 450
corresponds to an integer number of binary bits, meaning that the
number of voltage levels stored in memory cell 50 or 450 will be
equal to a power of two (e.g., 2, 4, 8, 16, etc.), though other
schemes are possible within the scope of the invention. Due to the
lower noise margins, it may be desirable to encode the data in
memory array 80 or 480 using any error correction code (ECC) known
in the art. In order to make the ECC more robust, the voltage
levels inside may be encoded in a non-binary order like, for
example, using a gray code to assign binary values to the voltage
levels. In the case of gray coding, only one bit changes in the
binary code for a single level increase or decrease in the voltage
level. Thus for an example a two bit gray encoding, the lowest
voltage level corresponding to the floating body region 24 voltage
being neutral might be encoded as logic-00, the next higher voltage
level being encoded as logic-01, the next higher voltage level
after that being encoded as logic-11, and the highest voltage level
corresponding to the maximum voltage level on floating body region
24 being encoded as logic-10. In an exemplary three bit gray
encoding, the logic levels from lowest to highest might be ordered
logic-000, logic-001, logic-011, logic-010, logic-110, logic-111,
logic-101, and logic-100. Since the most likely reading error is to
mistake one voltage level for one of the two immediately adjacent
voltage levels, this sort of encoding ensures that a single level
reading error will produce at most a single bit correction per
error minimizing the number of bits needing correction for any
single error in a single cell. Other encodings may be used, and
this example is in no way limiting.
A multi-level write operation can be performed using an alternating
write and verify algorithm, where a write pulse is first applied to
the memory cell 50 or 450, followed by a read operation to verify
if the desired memory state has been achieved. If the desired
memory state has not been achieved, another write pulse is applied
to the memory cell 50, followed by another read verification
operation. This loop is repeated until the desired memory state is
achieved.
For example, using band-to-band hot hole injection to write memory
cell 50 or 450, initially zero voltage is applied to BL terminal
74, zero voltage is applied to SL terminal 72, a negative voltage
is applied to WL terminal 70, and zero voltage is applied to the
substrate terminal 78. Then positive voltages of different
amplitudes are applied to BL terminal 74 to write different states
to floating body 24. This results in different floating body
potentials 24 corresponding to the different positive voltages or
the number of positive voltage pulses that have been applied to BL
terminal 74. Note that memory cell 50 must be written to the lowest
voltage state on floating body region 24 prior to executing this
algorithm.
In one particular non-limiting embodiment, the write operation is
performed by applying the following bias condition: a potential of
about 0.0 volts is applied to SL terminal 72, a potential of about
-1.2 volts is applied to WL terminal 70, and about 0.0 volts is
applied to substrate terminal 78, while the potential applied to BL
terminal 74 is incrementally raised. For example, in one
non-limiting embodiment, 25 millivolts is initially applied to BL
terminal 74, followed by a read verify operation. If the read
verify operation indicates that the cell current has reached the
desired state (i.e. cell current corresponding to whichever binary
value of 00, 01, 11 or 10 is desired is achieved), then the
multi-level write operation is successfully concluded. If the
desired state is not achieved, then the voltage applied to BL
terminal 74 is raised, for example, by another 25 millivolts, to 50
millivolts. This is subsequently followed by another read verify
operation, and this process iterates until the desired state is
achieved. However, the voltage levels described may vary from
embodiment to embodiment and the above voltage levels are exemplary
only and in no way limiting. To write four levels to the memory
cells, at least three different positive voltage pulses (which may
comprise of different amplitudes) to the BL terminal 74 are
required. The first pulse corresponds to writing the memory cell to
the level associated with the binary value of 01, the second pulse
corresponds to writing the memory cell to the level associated with
the binary value of 11, and the third pulse corresponds to writing
the memory cell to the level associated with the binary value of
10.
The write-then-verify algorithm is inherently slow since it
requires multiple write and read operations. The present invention
provides a multi-level write operation that can be performed
without alternate write and read operations as described in FIGS.
19A through 19F with respect to exemplary memory array 80. Persons
of ordinary skill in the art will appreciate that the principles
described will apply to all of the Half Transistor memory cells
within the scope of the present invention.
As shown in FIG. 19A, the potential of the floating body 24
increases over time as a result of hole injection to floating body
24, for example through an impact ionization mechanism. Once the
change in cell current reaches the level associated with the
desired state of the selected representative memory cell 50, the
voltage applied to BL terminal 74 can be removed. In this manner,
the multi-level write operation can be performed without alternate
write and read operations by applying a voltage ramp of the correct
duration. After the end of the pulse time, the applied voltage
returns to the starting value like, for example, ground. Thus as
shown in FIG. 19A, a voltage ramp of pulse width T1 applied to the
bit line terminal 74 of memory cell 50 in the lowest (logic-00
state) potential state will increase the potential of the floating
body 24 from the logic-00 level to the logic-01 level. Similarly, a
voltage ramp of pulse width T2 applied to the bit line terminal 74
of memory cell 50 in the lowest (logic-00 state) potential state
will increase the potential of the floating body 24 from the
logic-00 level to the logic-11 level, and a voltage ramp of pulse
width T3 applied to the bit line terminal 74 of memory cell 50 in
the lowest (logic-00 state) potential state will increase the
potential of the floating body 24 from the logic-00 level to the
logic-10 level.
In FIG. 19B this is accomplished in selected representative memory
cell 50a by ramping the voltage applied to BL terminal 74a, while
applying zero voltage to SL terminal 72a, a positive voltage to WL
terminal 70, and zero voltage to substrate terminal 78 of the
selected memory cells. These bias conditions will result in a hole
injection to the floating body 24 through an impact ionization
mechanism. The state of the memory cell 50a can be simultaneously
read for example by monitoring the change in the cell current
through read circuitry 91a coupled to the source line 72a.
In the rest of array 80, zero voltage is applied to the unselected
WL terminals 70b (not shown) through 70n, zero voltage is applied
to the unselected SL terminals 72b (not shown) through 72n, and
zero voltage is applied to the unselected BL terminals 74b through
74p. The cell current measured in the source line direction is the
total cell current of all memory cells 50 which share the same
source line 72a, but all of the unselected cells like
representative memory cell 50b are biased with zero voltage across
them from their bit line region 16 to their source line region 22
and do not conduct current as long as the source line terminal 72a
is correctly biased to maintain zero volts. As a result, only one
selected memory cell 50a sharing the same source line 72 can be
written at a time.
In FIG. 19B, the unselected representative memory cell 50b has zero
volts between the BL terminal 74p and the SL terminal 72a so no
current flows and the state of the data stored in them will not
change. Unselected representative memory cell 50c sharing BL
terminal 74a with selected representative memory cell 50a has its
WL terminal grounded. Thus its floating body region 24 does not get
the voltage coupling boost that the floating body region 24 in
selected representative memory cell 50a gets. A positive bias is
also applied to the unselected SL terminal 72n. This condition
substantially reduces the current in representative memory cell 50c
which reduces the degree of hole charge its floating body region 24
receives as the voltage applied to BL terminal 74a is ramped up.
Unselected representative memory cell 50d, sharing neither a row
nor a column with selected representative memory cell 50a, is shown
with gate 60 coupled to WL terminal 70n biased at 0.0V, bit line
region 16 coupled to BL terminal 74p biased at 0.0V, and buried
layer 22 coupled to source line terminal 72n biased at +1.2V. As
can be seen, these cells will be at holding mode. Memory cells in
state logic-1 will maintain the charge in floating body 24 because
the intrinsic bipolar device 30a will generate holes current to
replenish the charge in floating body 24, while memory cells in
state logic-0 will remain in neutral state.
FIG. 19B also shows reference generator circuits 93a through 93n
coupled respectively to source line terminals 72a through 72n and
read circuits 91a through 91n coupled respectively to source line
terminals 72a through 72n and coupled respectively to reference
generator circuit 93a through 93n. Reference generator circuit 93a
serves to store the initial total cell current of selected
representative memory cell 50a and provide this value to read
circuit 91a during the write operation in real time so that the
change in current can be monitored and feedback (not shown in FIG.
19B) can be used to shut off the voltage ramp at the appropriate
time. This function can be implemented in a variety of ways.
In FIG. 19C, for example, the cumulative charge of the initial
state for selected memory cell 50a sharing the same source line 72a
can be stored in a capacitor 97a. Transistor 95a is turned on when
charge is to be written into or read from capacitor 94.
Alternatively, as shown in FIG. 19D, reference cells 50Ra through
50Rn similar to a memory cell 50 replace capacitors 97a through 97n
in reference generator circuits 93a through 93n. The reference
cells 50Ra through 50Rn can also be used to store the initial state
of selected representative memory cell 50a.
In a similar manner, a multi-level write operation using an impact
ionization mechanism can be performed by ramping the write current
applied to BL terminal 74 instead of ramping the BL terminal 74
voltage.
In another embodiment, a multi-level write operation can be
performed on memory cell 50 through a band-to-band tunneling
mechanism by ramping the voltage applied to BL terminal 74, while
applying zero voltage to SL terminal 72, a negative voltage to WL
terminal 70, and zero voltage to substrate terminal 78 of the
selected memory cells 50. The unselected memory cells 50 will
remain in holding mode, with zero or negative voltage applied to WL
terminal 70, zero voltage applied to BL terminal 74, and a positive
voltage applied to SL terminal 72. Optionally, multiple BL
terminals 74 can be simultaneously selected to write multiple cells
in parallel. The potential of the floating body 24 of the selected
memory cell(s) 50 will increase as a result of the band-to-band
tunneling mechanism. The state of the selected memory cell(s) 50
can be simultaneously read for example by monitoring the change in
the cell current through a read circuit 91 coupled to the source
line. Once the change in the cell current reaches the desired level
associated with a state of the memory cell, the voltage applied to
BL terminal 74 can be removed. In this manner, the multi-level
write operation can be performed without alternate write and read
operations.
Similarly, the multi-level write operation using band-to-band
tunneling mechanism can also be performed by ramping the write
current applied to BL terminal 74 instead of ramping the voltage
applied to BL terminal 74.
In another embodiment, as shown in FIG. 19E, a read while writing
operation can be performed by monitoring the change in cell current
in the bit line direction through a reading circuit 99a coupled to
the bit line 74a. In some embodiments a reading circuit 99b through
99p (not shown in FIG. 19E) may be coupled to each bit of the other
bit lines 74b through 74p, while in other embodiments reading
circuit 99a may be shared between multiple columns using a decoding
scheme (not shown).
Reference cells 50R representing different memory states are used
to verify the state of the write operation. The reference cells 50R
can be configured through a write-then-verify operation, for
example, when the memory device is first powered up or during
subsequent refresh periods. Thus while selected representative
memory cell 50a is being written, selected reference cell 50R
containing the desired voltage state (or a similar voltage) to be
written is read and the value is used to provide feedback to read
circuit so that the write operation may be terminated when the
desired voltage level in selected representative memory cell 50a is
reached. In some embodiments, multiple columns of reference cells
containing different reference values corresponding to the
different multilevel cell write values may be present (not shown in
FIG. 19E).
In the voltage ramp operation, the resulting cell current of the
representative memory cell 50a being written is compared to the
reference cell 50R current by means of the read circuitry 99a.
During this read while writing operation, the reference cell 50R is
also being biased at the same bias conditions applied to the
selected memory cell 50 during the write operation. Therefore, the
write operation needs to be ceased after the desired memory state
is achieved to prevent altering the state of the reference cell
50R.
As shown in FIG. 19F, for the current ramp operation, the voltage
at the bit line 74a can be sensed instead of the cell current. In
the current ramp operation, a positive bias is applied to the
source line terminal 72a and current is forced through the BL
terminal 74a. The voltage of the BL terminal 74a will reflect the
state of the memory cell 50a. Initially, when memory cell 50a is in
logic-0 state, a large voltage drop is observed across the memory
cell 50a and the voltage of the BL terminal 74a will be low. As the
current flow through the memory cell 50a increases, hole injection
will increase, resulting memory cell 50a to be in logic-1 state. At
the conclusion of the logic-1 state write operation, the voltage
drop across the memory cell 50a will decrease and an increase in
the potential of BL terminal 74a will be observed.
An example of a multi-level write operation without alternate read
and write operations, using a read while programming
operation/scheme in the bit line direction is given, where two bits
are stored per memory cell 50, requiring four states to be storable
in each memory cell 50.
With increasing charge in the floating body 24, the four states are
referred to as states "00", "01", "10", and "11". To program a
memory cell 50a to a state "01", the reference cell 50R
corresponding to state "01" is activated. Subsequently, the bias
conditions described above are applied both to the selected memory
cell 50 and to the "01" reference cell 50R: zero voltage is applied
to the source line terminal 72, zero voltage is applied to the
substrate terminal 78, a positive voltage is applied to the WL
terminal 70 (for the impact ionization mechanism), while the BL
terminal 74 is being ramped up, starting from zero voltage.
Starting the ramp voltage from a low voltage (i.e. zero volts)
ensures that the state of the reference cell 50R does not
change.
The voltage applied to the BL terminal 74a is then increased.
Consequently, holes are injected into the floating body 24 of the
selected cell 50 and subsequently the cell current of the selected
cell 50 increases. Once the cell current of the selected cell 50
reaches that of the "01" reference cell, the write operation is
stopped by removing the positive voltage applied to the BL terminal
74 and WL terminal 70.
Unselected representative memory cell 50b, which shares a row with
selected representative memory cell 50a, has its bipolar device 30
turned off because there is no voltage between the collector and
emitter terminals. It retains its logic state during the short
duration of the multi-level write operation.
Unselected representative memory cell 50c, which shares a column
with selected representative memory cell 50a, is in the holding
state. Less base current will flow into the floating body 24 due to
the smaller potential difference between SL terminal 72n and BL
terminal 74a (i.e. the emitter and collector terminals of the n-p-n
bipolar device 30). It also retains its logic state during the
short duration of the multi-level write operation.
Unselected representative memory cell 50d, which shares neither a
row nor a column with selected representative memory cell 50a, is
in the holding state. It too retains its logic state during the
short duration of the multi-level write operation.
It is noteworthy that the holding operation for memory cell 50 in
multistate mode is self selecting. In other words, the quantity of
holes injected into the floating body 24 is proportional to the
quantity of holes (i.e., the charge) already present on the
floating body 24. Thus each memory cell selects its own correct
degree of holding current.
FIGS. 20 and 21 show gated half transistor memory cell 550 with
FIG. 21 showing the top view of the memory cell 550 shown in FIG.
20. Referring now to both FIGS. 20 and 21, reference numbers
previously referred to in earlier drawing figures have the same,
similar, or analogous functions as in the earlier described
embodiments. Memory cell 550 has a fin structure 52 fabricated on
substrate 12, so as to extend from the surface of the substrate to
form a three-dimensional structure, with fin 52 extending
substantially perpendicular to and above the top surface of the
substrate 12. Fin structure 52 is conductive and is built on buried
well layer 22 which is itself built on top of substrate 12.
Alternatively, buried well 22 could be a diffusion inside substrate
12 with the rest of the fin 52 constructed above it, or buried well
22 could be a conductive layer on top of substrate 12 connected to
all the other fin 52 structures in a manner similar to memory cell
450 described above. Fin 52 is typically made of silicon, but may
comprise germanium, silicon germanium, gallium arsenide, carbon
nanotubes, or other semiconductor materials known in the art.
Buried well layer 22 may be formed by an ion implantation process
on the material of substrate 12 which may be followed by an etch so
that buried well 22 is above the portion of substrate 12 remaining
after the etch. Alternatively, buried well layer 22 may be grown
epitaxially above substrate 22 and the unwanted portions may then
be etched away. Buried well layer 22, which has a second
conductivity type (such as n-type conductivity type), insulates the
floating body region 24, which has a first conductivity type (such
as p-type conductivity type), from the bulk substrate 12 also of
the first conductivity type. Fin structure 52 includes bit line
region 16 having a second conductivity type (such as n-type
conductivity type). Memory cell 550 further includes gates 60 on
two opposite sides of the floating substrate region 24 insulated
from floating body 24 by insulating layers 62. Gates 60 are
insulated from floating body 24 by insulating layers 62. Gates 60
are positioned between the bit line region 16 and the insulating
layer 28, adjacent to the floating body 24.
Thus, the floating body region 24 is bounded by the top surface of
the fin 52, the facing side and bottom of bit line region 16, top
of the buried well layer 22, and insulating layers 26, 28 and 62.
Insulating layers 26 and 28 insulate cell 550 from neighboring
cells 550 when multiple cells 550 are joined to make a memory
array. Insulating layer 26 insulates adjacent buried layer wells
22, while insulating layer 28 does not. Thus the buried layer 22 is
therefore continuous (i.e. electrically conductive) in one
direction. In this embodiment, the surface 14 of the semiconductor
is at the level of the top of the fin structure. As in other
embodiments, there is no contact to the buried layer 22 at the
semiconductor surface 14 inside the boundary of memory cell
550.
As shown in FIG. 22, an alternate fin structure 52A can be
constructed. In this embodiment, gates 60 and insulating layers 62
can enclose three sides of the floating substrate region 24. The
presence of the gate 60 on three sides allows better control of the
charge in floating body region 24.
Memory cell 550 can be used to replace memory cell 50 in an array
similar to array 80 having similar connectivity between the cells
and the array control signal terminals. In such a case, the hold,
read and write operations are similar to those in the lateral
device embodiments described earlier for memory cell 50 in array
80. As with the other embodiments, the first and second
conductivity types can be reversed as a matter of design choice. As
with the other embodiments, many other variations and combinations
of elements are possible, and the examples described in no way
limit the present invention.
FIG. 22B shows an array 580 of memory cells 550. Due the nature of
fin structure 52A, the most compact layout will typically be with
the word lines 70 running perpendicular to the source lines 72,
instead of in parallel as in memory array 80 discussed above. This
leads to the structure of array 580 where the cell 550 is
constructed using fin structure 52A and the source lines 72a
through 72p run parallel to the bit lines 74a through 74p and
orthogonal to the word lines 70a through 70m. The operation of
memory array 580 is described in commonly assigned U.S. patent
application entitled "COMPACT SEMICONDUCTOR MEMORY DEVICE HAVING
REDUCED NUMBER OF CONTACTS, METHODS OF OPERATING AND METHODS OF
MAKING", Ser. No. 12/897,528, filed on Oct. 4, 2010, the content of
which is incorporated by reference above.
FIG. 23A shows another embodiment of a gated half transistor memory
cell 150 (denoted by a dotted line) according to the present
invention. FIG. 23B shows a smaller portion of FIG. 23A comprising
a single memory cell 150 with two cross section lines I-I' and
II-II'. FIG. 23C shows the cross section designated I-I' in FIG.
23B. FIG. 23D shows the cross section designated II-II' in FIG.
23B. Present in FIGS. 23A through 23F are substrate 12,
semiconductor surface 14, bit line region 16, buried well layer 22,
floating body region 24, insulating layers 26 and 28, gate 60, gate
insulator 62, word line terminal 70, buried well terminal 72, bit
line terminal 74 and substrate terminal 78, all of which perform
similar functions in the exemplary embodiments of memory cell 150
as they did in the exemplary embodiments of memory cell 50
described above.
Referring now to FIGS. 23A, 23B, 23C and 23D, the cell 150 includes
a substrate 12 of a first conductivity type, such as a p-type
conductivity type, for example. Substrate 12 is typically made of
silicon, but may comprise germanium, silicon germanium, gallium
arsenide, carbon nanotubes, or other semiconductor materials known
in the art. A buried layer 22 of the second conductivity type is
provided in the substrate 12. Buried layer 22 is also formed by an
ion implantation process on the material of substrate 12.
Alternatively, buried layer 22 can also be grown epitaxially.
A bit line region 16 having a second conductivity type, such as
n-type, for example, is provided in floating body 24 and is exposed
at surface 14. Bit line region 16 is formed by an implantation
process formed on the material making up floating body 24,
according to any of implantation processes known and typically used
in the art. Alternatively, a solid state diffusion process could be
used to form bit line region 16.
A floating body region 24 of the substrate 12 is bounded by surface
14, bit line region 16, insulating layers 26 and 28 and buried
layer 22. Insulating layers 26 and 28 (e.g., shallow trench
isolation (STI)), may be made of silicon oxide, for example.
Insulating layers 26 and 28 insulate cell 150 from neighboring
cells 150 when multiple cells 150 are joined in an array 180 to
make a memory device as illustrated in FIG. 25. Insulating layer 26
insulates both neighboring body regions 24 and buried regions 22 of
adjacent cells memory cells 150A, 150, and 150B, while insulating
layer 28 insulates neighboring body regions 24, but not neighboring
buried layer regions 22, allowing the buried layer 22 to be
continuous (i.e. electrically conductive) in one direction in
parallel with the II-II' cut line as shown in FIGS. 23B and 23D. As
in other embodiments, there is no contact to the buried layer 22 at
the semiconductor surface 14 inside the boundary of memory cell
150.
A gate 60 is positioned in between bit line regions 16 of
neighboring cells 150 and 150A and above the surface 14, the
floating body regions 24, and one of the adjacent insulating layers
26 as shown in FIG. 23C. In this arrangement, the gate terminal 70
is coupled to the gates 60 of both memory cells 150 and 150A. The
gate 60 is insulated from surface 14 by an insulating layer 62.
Insulating layer 62 may be made of silicon oxide and/or other
dielectric materials, such as, but not limited to, tantalum
peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or
aluminum oxide. The gate 60 may be made of polysilicon material or
metal gate electrode, such as tungsten, tantalum, titanium and
their nitrides. In FIGS. 23A, 23B and 23C, the gate 60 is shown
above the insulating layer 26 isolating neighboring cells 150 and
150A.
Cell 150 further includes word line (WL) terminal 70 electrically
connected to gate 60, bit line (BL) terminal 74 electrically
connected to bit line region 16, source line (SL) terminal 72
electrically connected to the buried layer 22, and substrate
terminal 78 electrically connected to substrate 12.
As shown in FIG. 23E, contact to buried well region 22 can be made
through region 20 having a second conductivity type, and which is
electrically connected to buried well region 22 and buried well
terminal 72, while contact to substrate region 12 can be made
through region 28 having a first conductivity type, and which is
electrically connected to substrate region 12 and substrate
terminal 78. The SL terminal 72 serves as the back bias terminal
for the memory cell 150.
As shown in FIG. 23F, the buried well 22 (and subsequently SL
terminal 72) may also be shared between two adjacent memory cells
150 and 150B not sharing the same WL terminal 70. In this
embodiment, insulating layer 26A is built to a similar depth as
insulating layer 28 allowing this connection to be made using
buried well 22. Thus when a plurality of memory cells 150 are
arranged in an array the source line terminals 72 are shared
between pairs of adjacent rows of cells 150 and the word line
terminals 70 are shared between pairs of adjacent rows that are
offset by one row from the pairs of rows sharing source line
terminal 72. Thus each memory cell 150 shares a source line
terminal with one adjacent cell (e.g., 150B) and a word line
terminal 70 with another adjacent cell (e.g., 150A). It is worth
noting that this connectivity is possible because when memory cells
150 are mirrored in alternate rows when arrayed, while memory cell
50 is not mirrored when arrayed.
FIGS. 24A through 24E shown an alternate embodiment of memory cell
150 where a part of the gate 60 can also be formed inside a trench
adjacent to the floating body regions 24 of two adjacent memory
cells 150. The primary difference between this embodiment and the
one described in FIGS. 23A through 23E is that the insulating
layers 26 in alternate rows adjacent to the floating body regions
24 and under the gates 60 are replaced with a trench labeled 26T in
FIG. 24C. This trench can be filled with gate insulator 62 and gate
material 60 to form a "T" shaped structure. This allows gate 60 to
be adjacent to floating body region 24 on two sides allowing better
control of the charge in floating body region 24 in response to
electrical signals applied to gate 60 through word line terminal
70. In particular, operations where word line terminal is driven to
a positive voltage potential to provide a boost to the voltage
potential of the floating body 24 by means of capacitive coupling
will benefit from this arrangement. since the capacitance between
the gate 60 and the floating body 24 will be substantially
increased.
FIG. 24A shows a top view of one such embodiment of a memory cell
150 (denoted by a dotted line) according to the present invention.
FIG. 24B shows a smaller portion of FIG. 24A with two cross section
lines I-I' and II-II'. FIG. 24C shows the cross section designated
I-I' in FIG. 24B. FIG. 24D shows the cross section designated
II-II' in FIG. 24B. Present in FIGS. 24A through 24F are substrate
12, semiconductor surface 14, region 16, buried well layer 22,
floating body region 24, insulating layers 26 and 28, gate 60, gate
insulator 62, word line terminal 70, buried well terminal 72, bit
line terminal 74 and substrate terminal 78, all of which perform
similar functions in this exemplary embodiment as they did in the
earlier exemplary embodiments of memory cell 150 described
above.
Referring now to FIGS. 24A, 24B, 24C and 24D, the cell 150 includes
a substrate 12 of a first conductivity type, such as a p-type
conductivity type, for example. Substrate 12 is typically made of
silicon, but may comprise germanium, silicon germanium, gallium
arsenide, carbon nanotubes, or other semiconductor materials known
in the art. A buried layer 22 of the second conductivity type is
provided in the substrate 12. Buried layer 22 is also formed by an
ion implantation process on the material of substrate 12.
Alternatively, buried layer 22 can also be grown epitaxially.
A region 16 having a second conductivity type, such as n-type, for
example, is provided in floating body 24 and is exposed at surface
14. Region 16 is formed by an implantation process formed on the
material making up floating body 24, according to any of
implantation processes known and typically used in the art.
Alternatively, a solid state diffusion process could be used to
form region 16.
A floating body region 24 of the substrate 12 is bounded by surface
14, region 16, insulating layers 26, and 28, buried layer 22, and
trench 26T. Insulating layers 26 and 28 (e.g., shallow trench
isolation (STI)), may be made of silicon oxide, for example.
Insulating layers 26 and 28 combined with trench 26T insulate cell
150 from neighboring cells 150 when multiple cells 150 are joined
in an array 180 to make a memory device as illustrated in FIG. 25.
Insulating layer 26 and trench 26T together insulate both
neighboring body regions 24 and buried regions 22 of adjacent cells
memory cells 150A, 150, and 150B, while insulating layer 28
insulates neighboring body regions 24, but not neighboring buried
layer regions 22, allowing the buried layer 22 to be continuous
(i.e. electrically conductive) in one direction in parallel with
the II-II' cut line as shown in FIGS. 24B and 24D.
A gate 60 is positioned in trench 26T in between bit line regions
16 of neighboring cells 150 and 150A and above the surface 14 over
the floating body regions 24 forming a "T" shaped structure as
shown in FIG. 24C. In this arrangement, the gate terminal 70 is
coupled to the gates 60 of both memory cells 150 and 150A. The gate
60 is insulated from floating body regions 24 by an insulating
layer 62 both on surface 14 and along the walls and bottom of
trench 26T. Insulating layer 62 may be made of silicon oxide and/or
other dielectric materials, such as, but not limited to, tantalum
peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or
aluminum oxide. The gate 60 may be made of polysilicon material or
metal gate electrode, such as tungsten, tantalum, titanium and
their nitrides. The trench 26T could be formed through silicon
etching process similar to the STI formation after the STI 26 and
28 have been formed. Instead of filling the trench 26T with thick
oxide, gate oxide 62 could be grown after the trench etch, followed
by gate 60 formation.
Cell 150 further includes word line (WL) terminal 70 electrically
connected to gate 60, bit line (BL) terminal 74 electrically
connected to region 16, source line (SL) terminal 72 electrically
connected to the buried layer 22, and substrate terminal 78
electrically connected to substrate 12.
As shown in FIG. 24E, contact to buried well region 22 can be made
through region 20 having a second conductivity type, and which is
electrically connected to buried well region 22 and buried well
terminal 72, while contact to substrate region 12 can be made
through region 28 having a first conductivity type, and which is
electrically connected to substrate region 12 and substrate
terminal 78. The SL terminal 72 serves as the back bias terminal
for the memory cell 150.
As shown in FIG. 24F, the buried well 22 (and subsequently SL
terminal 72) may also be shared between two adjacent memory cells
150 and 150B not sharing the same WL terminal 70. In this
embodiment, insulating layer 26A is built to a similar depth as
insulating layer 28 allowing this connection to be made using
buried well 22. Thus when a plurality of memory cells 150 are
arranged in an array the source line terminals 72 are shared
between pairs of adjacent rows of cells 150 and the word line
terminals 70 are shared between pairs of adjacent rows that are
offset by one row from the pairs of rows sharing source line
terminal 72. Thus each memory cell 150 shares a source line
terminal with one adjacent cell (e.g., 150B) and a word line
terminal 70 with another adjacent cell (e.g., 150A). It is worth
noting that this connectivity is possible because when memory cells
150 are mirrored in alternate rows when arrayed, while memory cell
50 is not mirrored when arrayed.
Persons of ordinary skill in the art will appreciate that many
other embodiments of the memory cell 150 other than the exemplary
embodiments described in conjunction with FIGS. 23A through 24F are
possible. For example, the first and second conductivity types may
be reversed as a matter of design choice. Other physical geometries
may be used like, for example, substrate 12 may be replaced by a
well placed in a substrate of the second conductivity type (not
shown) as a matter of design choice. Thus the embodiments shown are
in no way limiting of the present invention.
FIG. 25A shows an exemplary memory array 180 of memory cells 150.
In the exemplary array 180 an embodiment of memory cell 150 is
chosen where word lines 70a through 70n are shared between adjacent
rows of memory cells 150 and source lines 72a through 72n+1 are
shared between adjacent rows of memory cells 150 offset by one row.
Thus there is one more source line 72 than there are row lines 70
because the top and bottom rows do not have an adjacent row of
memory cells 150 to share source lines 72 with. Because the WL
terminals 70a through 70n and source line terminals 72a through
72n+1 can be shared between neighboring memory cells, a smaller
memory array 180 may be realized since the effective size of memory
cell 150 is reduced due the shared features. Alternatively, the
memory array 180 of memory cells 150 can be arranged with one more
word line 70 than there are source lines 72 with the top and bottom
rows each not sharing word line 70 with adjacent rows.
As shown in FIG. 25B, the circuit schematic for an individual
memory cell 150 is identical to that for memory cell 50 as shown in
FIG. 3A, the main differences between memory cells 50 and 150 being
the physical construction, relative orientation, and the sharing of
control lines. Thus the operating principles of memory cell 150
will follow the principles of the previously described memory cell
50. The memory cell operations will be described, realizing that
the WL and SL terminals are now shared between neighboring memory
cells. Persons of ordinary skill in the art will realize the
operation of the embodiments of memory cell 150 which share word
lines 70 but have separate source lines 72 can be handled
identically by manipulating the non-shared source lines 72
identically or by manipulating them in an analogous manner to other
rows in the memory array as a matter of design choice.
As illustrated in FIG. 26, the holding operation for memory cell
150 can be performed in a similar manner to that for memory cell 50
by applying a positive bias to the back bias terminal (i.e. SL
terminal 72 coupled to buried well region 22) while grounding bit
line terminal 74 coupled to bit line region 16 and substrate
terminal 78 coupled to substrate 12. As previously described, the
holding operation is relatively independent of the voltage applied
to terminal 70 which is preferably grounded in some embodiments.
Inherent in the memory cell 150 is n-p-n bipolar device 30 formed
by buried well region 22, floating body 24, and bit line region
16.
If floating body 24 is positively charged (i.e. in a logic-1
state), the bipolar transistor 30 formed by bit line region 16,
floating body 24, and buried well region 22 will be turned on as
discussed above in conjunction with FIGS. 3A through 3C above. A
fraction of the bipolar transistor current will then flow into
floating body region 24 (usually referred to as the base current)
and maintain the logic-1 data. The efficiency of the holding
operation can be enhanced by designing the bipolar device formed by
buried well region 22, floating region 24, and bit line region 16
to be a low-gain bipolar device, where the bipolar gain is defined
as the ratio of the collector current flowing out of SL terminal 72
to the base current flowing into the floating region 24.
For memory cells in the logic-0 state, the bipolar device will not
be turned on, and consequently no base hole current will flow into
floating body region 24 as discussed above in conjunction with
FIGS. 3A through 3C above. Therefore, memory cells in state logic-0
will remain in state logic-0.
A periodic pulse of positive voltage can be applied to the SL
terminal 72 as opposed to applying a constant positive bias to
reduce the power consumption of the memory cell 150 in a manner
analogous to that described in conjunction with FIGS. 4A through 4D
above.
As illustrated in FIG. 26, an example of the bias condition for a
two row holding operation is applied to exemplary memory array 180.
In one particular non-limiting embodiment, about +1.2 volts is
applied to SL terminal 72b, about 0.0 volts is applied to the other
source line terminals 72a and 72c (not shown) through 72n+1, about
0.0 volts is applied to BL terminals 74a through 74p, about 0.0
volts is applied to WL terminals 70a through 70n, and about 0.0
volts is applied to substrate terminals 78a through 78n+1. This
will place representative memory cells 150a, 150c, 150d, and 150f
into a holding state. These voltage levels are exemplary only may
vary substantially as a matter of design choice and processing
technology node and are in no way limiting.
As illustrated in FIGS. 27 and 28A through 28P, the charge stored
in the floating body 24 can be sensed by monitoring the cell
current of the memory cell 150. If cell 150 is in a state logic-1
having holes in the body region 24, then the memory cell will have
a higher cell current, compared to if cell 150 is in a state
logic-0 having no holes in body region 24. A sensing circuit
typically connected to BL terminal 74 of memory array 180 can then
be used to determine the data state of the memory cell. Examples of
the read operation are described with reference to Yoshida,
Ohsawa-1, and Ohsawa-2 discussed above.
The read operation can be performed by applying the following bias
condition to memory cell 150: a positive voltage is applied to the
selected BL terminal 74, and a positive voltage greater than the
positive voltage applied to the selected BL terminal 74 is applied
to the selected WL terminal 70, zero voltage is applied to the
selected SL terminal 72, and zero voltage is applied to the
substrate terminal 78. The unselected BL terminals will remain at
zero voltage, the unselected WL terminals will remain at zero
voltage, and the unselected SL terminals will remain at positive
voltage.
The bias conditions for an exemplary embodiment for a read
operation for the exemplary memory array 180 are shown in FIG. 27,
while the bias conditions during a read operation for selected
representative memory cell 150a are further illustrated in FIGS.
28A through 28B and the bias conditions during a read operation for
the seven cases illustrated by unselected representative memory
cells 150b through 150h during read operations are further shown in
FIGS. 28C through 28P. In particular, the bias conditions for
unselected representative memory cell 150b sharing the same WL
terminal 70a and BL terminal 74a but not the same SL terminal 72 as
the selected representative memory cell 150a are shown in FIGS. 28C
through 28D. The bias conditions for unselected representative
memory cell 150c sharing the same SL terminal 72b and BL terminal
74a but not the same WL terminal 70 as the selected representative
memory cell 150a are shown in FIGS. 28E through 28F. The bias
conditions for unselected representative memory cell 150d sharing
the same WL terminal 70a and SL terminal 72b but not the same BL
terminal 74 as the selected representative memory cell 150a are
shown in FIGS. 28G through 28H. FIGS. 281 through 28J show the bias
conditions for unselected representative memory cell 150e sharing
the same WL terminal 70a but neither the same SL terminal 72 nor BL
terminal 74 as the selected representative memory cell 150a. FIGS.
28K through 28L show the bias conditions for unselected
representative memory cell 150f sharing the same SL terminal 72b
but neither the same WL terminal 70 nor BL terminal 74 as the
selected representative memory cell 150a. The bias conditions for
unselected representative memory cell 150g sharing the same BL
terminal 74a as the selected representative memory cell 150a but
not the same WL terminal 70 nor SL terminal 72 is shown in FIGS.
28M through 28N. The bias condition for representative memory cell
150h not sharing any control terminals as the selected
representative memory cell 150a is shown in FIGS. 28O through
28P.
In one particular non-limiting and exemplary embodiment illustrated
in FIGS. 27, 28A and 28B, the bias conditions for selected
representative memory cell 150a and are shown. In particular, about
0.0 volts is applied to the selected SL terminal 72b, about +0.4
volts is applied to the selected bit line terminal 74a, about +1.2
volts is applied to the selected word line terminal 70a, and about
0.0 volts is applied to substrate terminal 78 (not shown in FIG.
28B).
In the remainder of exemplary array 180, the unselected bit line
terminals 74b through 74p remain at 0.0 volts, the unselected word
line terminals 70b through 70n remain at 0.0 volts, and the
unselected SL terminals 72a and 72c (not shown in FIG. 27) through
72n+1 remain at +1.2 volts. FIGS. 28C through 28P show in more
detail the unselected representative memory cells 150b-150h in
memory array 180. It is noteworthy that these voltage levels are
exemplary only may vary substantially as a matter of design choice
and processing technology node, and are in no way limiting.
As shown in FIGS. 27, 28C and 28D, representative memory cell 150b
sharing the same WL terminal 70a and BL terminal 74a but not the
same SL terminal 72 as the representative selected memory cell
150a, both the BL and SL terminal are positively biased. The
potential difference between the BL and SL terminals (i.e. the
emitter and collector terminals of the bipolar device 30) is lower
compared to the memory cells in the holding mode, reducing the base
current flowing to the floating body 24. However, because read
operation is accomplished much faster (in the order of nanoseconds)
compared to the lifetime of the charge in the floating body 24 (in
the order of milliseconds), it should cause little disruptions to
the charge stored in the floating body.
As shown in FIGS. 27, 28E and 28F, representative memory cell 150c
sharing the same SL terminal 72b and BL terminal 74a but not the
same WL terminal 70 as the selected representative memory cell
150a, both the WL terminal 72b and the SL terminal 72 are grounded
with the BL terminal positively biased. As a result, memory cell
150c will still be at holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate hole current to replenish
the charge in floating body 24; while memory cells in state logic-0
will remain in neutral state.
As shown in FIGS. 27, 28G and 28H, representative memory cell 150d
sharing the same WL terminal 70a and SL terminal 72b but not the
same BL terminal 74 as the selected representative memory cell
150a, both the SL terminal 72b and BL terminal 74b are grounded
with the WL terminal 70a at +1.2V. As a result, there is no
potential difference between the emitter and collector terminals of
the n-p-n bipolar device 30 and consequently representative memory
cell 150d is no longer in holding mode. However, because read
operation is accomplished much faster (in the order of nanoseconds)
compared to the lifetime of the charge in the floating body 24 (in
the order of milliseconds), it should cause little disruptions to
the charge stored in the floating body.
As shown in FIGS. 27, 281 and 28J, representative memory cell 150e
sharing the same WL terminal 70a but not the same SL terminal 72
nor BL terminal 74 as the selected representative memory cell 150a,
the SL terminal remains positively biased. As a result, memory cell
150e will still be at holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate holes current to
replenish the charge in floating body 24; while memory cells in
state logic-0 will remain in the neutral state.
As shown in FIGS. 27, 28K and 28L, representative memory cell 150f
sharing the same SL terminal 72b but not the same WL terminal 70
nor BL terminal 74 as the selected representative memory cell 150a,
both the SL terminal 72 and BL terminal 74 are now grounded. As a
result, there is no potential difference between the emitter and
collector terminals of the n-p-n bipolar device 30 and consequently
memory cells 150f is no longer in holding mode. However, because
read operation is accomplished much faster (in the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (in the order of milliseconds), it should cause little
disruptions to the charge stored in the floating body.
As shown in FIGS. 27, 28M and 28N, representative memory cell 150g
sharing the same BL terminal 74a but not the same WL terminal 70
nor SL terminal 72 as the selected representative memory cell 150a,
a positive voltage is applied to the BL terminal 74. Less base
current will flow into the floating body 24 due to the smaller
potential difference between SL terminal 72 and BL terminal 74
(i.e. the emitter and collector terminals of the n-p-n bipolar
device 30). However, because read operation is accomplished much
faster (in the order of nanoseconds) compared to the lifetime of
the charge in the floating body 24 (in the order of milliseconds),
it should cause little disruptions to the charge stored in the
floating body.
As shown in FIGS. 27, 28O and 28P, representative memory cells 150h
not sharing WL, BL, and SL terminals as the selected representative
memory cell 150a, both the SL terminal 72 will remain positively
charged and the BL terminal remain grounded (FIGS. 28O-28P). As can
be seen, these cells will be at holding mode, where memory cells in
state logic-1 will maintain the charge in floating body 24 because
the intrinsic bipolar device 30 will generate holes current to
replenish the charge in floating body 24; while memory cells in
state logic-0 will remain in the neutral state.
It is noteworthy that the voltage levels described in all the
different cases above are exemplary only may vary substantially
from embodiment to embodiment as a matter of both design choice and
processing technology node, and are in no way limiting.
A two row write logic-0 operation of the cell 150 is now described
with reference to FIG. 29. A negative bias may be applied to the
back bias terminal (i.e. SL terminal 72), zero potential may be
applied to WL terminal 70, zero voltage may be applied to BL
terminal 72 and substrate terminal 78. The unselected SL terminal
72 will remain positively biased. Under these conditions, the p-n
junction between floating body 24 and buried well 22 of the
selected cell 50 is forward-biased, evacuating any holes from the
floating body 24. In one particular non-limiting embodiment, about
-0.5 volts is applied to terminal 72, about 0.0 volts is applied to
terminal 70, and about 0.0 volts is applied to terminal 74 and 78.
However, these voltage levels may vary, while maintaining the
relative relationship between the charges applied, as described
above.
In FIG. 29, the selected SL terminal 72b is biased at about -0.5V
while the unselected SL terminals 72a, and 72c (not shown) through
72n+1 are biased at about +1.2V, the WL terminals 70a through 70n
are biased at about 0.0V, the BL terminals 74a through 74p are
biased at about 0.0V and the substrate terminals 78a through 78n+1
are biased at about 0.0V. In some embodiments where the substrate
is really a well in another substrate (not shown), the substrate
terminals may be biased at about -0.5V to avoid unwanted current
from the selected SL terminal 72b. This condition causes all of the
memory cells 150 coupled to SL terminal 72b, including the selected
representative memory cells 150a, 150c, 150d, and 150f, to be
written to the logic-0 state.
FIGS. 29, 29A and 29B show an example of bias conditions for the
selected and unselected memory cells 150 during a two row write
logic-0 operation in memory array 180. For the selected memory
cells, including representative memory cells 150a, 150c, 150d and
150f, the negative bias applied to SL terminal 72 causes large
potential difference between floating body 24 and buried well
region 22. This causes the hole charge in the floating body 24 to
be discharged as discussed above. Because the buried well 22 is
shared among multiple memory cells 50, all memory cells 150 sharing
the same SL terminal 72 will be written into state logic-0.
An example of bias conditions and an equivalent circuit diagram
illustrating the intrinsic n-p-n bipolar devices 30 of unselected
memory cells 150, including representative memory cells 150b, 150e,
150g and 150h, during write logic-0 operations are illustrated in
FIGS. 29A through 29B. Since the write logic-0 operation only
involves a negative voltage to the selected SL terminal 72, the
bias conditions for all the unselected cells are the same. As can
be seen, the unselected memory cells will be in a holding
operation, with the BL terminal at about 0.0 volts, WL terminal at
zero or negative voltage, and the unselected SL terminal positively
biased.
As illustrated in FIG. 30, a single column write logic-0 operation
can be performed by applying a negative bias to the BL terminal 74
as opposed to the SL terminal 72 (as in FIGS. 29, 29A, and 29B).
The SL terminal 72 will be positively biased, while zero voltage is
applied to the substrate terminal 78, and zero voltage is applied
to the WL terminal 70. Under these conditions, all memory cells
sharing the same BL terminal 74 will be written into state logic-0
while all the other memory cells 150 in the array 180 will be in
the holding state.
In FIG. 30, selected BL terminal 74a may be biased at about -1.2V
while the unselected BL terminals 74b through 74p may be biased at
about 0.0V, the WL terminals 70a through 70n may be biased at about
0.0V, the source line terminals 72a through 27n+1 may be biased at
+1.2V, and the substrate terminals 78a through 78n+1 may be biased
at 0.0V. This condition causes all of the memory cells 150 coupled
to BL terminal 74a, including the selected representative memory
cells 150a, 150b, 150c, and 150g, to be written to the logic-0
state while the remaining memory cells 150, including unselected
representative memory cells 150d, 150e, 150f, and 150h, to be in a
holding operation. These voltage levels are exemplary only may vary
substantially from embodiment to embodiment as a matter of design
choice and processing technology node used, and are in no way
limiting.
As illustrated in FIGS. 31 and 32A through 32P, a single cell write
logic-0 operation that allows for individual bit writing can be
performed by applying a positive voltage to WL terminal 70, a
negative voltage to BL terminal 74, zero or positive voltage to SL
terminal 72, and zero voltage to substrate terminal 78. Under these
conditions, the floating body 24 potential will increase through
capacitive coupling from the positive voltage applied to the WL
terminal 70. As a result of the floating body 24 potential increase
and the negative voltage applied to the BL terminal 74, the p-n
junction between floating body 24 and bit line region 16 is
forward-biased, evacuating any holes from the floating body 24. To
reduce undesired write logic-0 disturb to other memory cells 150 in
the memory array 180, the applied potential can be optimized as
follows: if the floating body 24 potential of state logic-1 is
referred to V.sub.FB1, then the voltage applied to the WL terminal
70 is configured to increase the floating body 24 potential by
V.sub.FB1/2 while -V.sub.FB1/2 is applied to BL terminal 74.
In one particular non-limiting embodiment, the following bias
conditions are applied to the selected memory cell 150a: a
potential of about 0.0 volts to SL terminal 72b, a potential of
about -0.2 volts to BL terminal 74a, a potential of about +0.5
volts is applied to WL terminal 70a, and about 0.0 volts is applied
to substrate terminals 78a through 78n+1; while about +1.2 volts is
applied to unselected SL terminals 72a and 72c (not shown) through
72n+1, about 0.0 volts is applied to unselected BL terminals 74b
through 74p, and about 0.0 volts is applied to unselected WL
terminals 70b through 70n. FIG. 31 shows the bias condition for the
selected and unselected memory cells 150 in memory array 180.
However, these voltage levels are exemplary only may vary
substantially from embodiment to embodiment as a matter of design
choice and processing technology node used, and are in no way
limiting.
The bias conditions of the selected representative memory cell 150a
under write logic-0 operation are further elaborated and are shown
in FIGS. 32A through 32B. As discussed above, the potential
difference between floating body 24 and bit line region 16
(connected to BL terminal 74a) is now increased, resulting in a
higher forward bias current than the base hole current generated by
the n-p-n bipolar devices 30 formed by buried well region 22,
floating body 24, and bit line region 16. The net result is that
holes will be evacuated from floating body 24.
The unselected memory cells 150 during write logic-0 operations are
shown in FIGS. 32C through 32P: The bias conditions for memory cell
150b sharing the same WL terminal 70a and BL terminal 74a but not
the same SL terminal 72 as the selected memory cell 150a are shown
in FIGS. 32C through 32D. The bias conditions for memory cell 150c
sharing the same SL terminal 72b and BL terminal 74a but not the
same WL terminal 70 as the selected memory cell 150a are shown in
FIGS. 32E through 32F. The bias conditions for memory cell 150d
sharing the same WL terminal 70a and SL terminal 72b but not the
same BL terminal 74 as the selected memory cell 150 are shown in
FIGS. 32G through 32H. FIGS. 321 through 32J show the bias
conditions for memory cell 150e sharing the same WL terminal 70a
but not the same SL terminal 72 nor BL terminal 74 as the selected
memory cell 150a. FIGS. 32K through 32L show the bias conditions
for memory cell 150f sharing the same SL terminal 72b but not the
same WL terminal 70 nor BL terminal 74 as the selected memory cell
150a. The bias conditions for memory cells sharing the same BL
terminal 74a as the selected memory cell 150a but not the same WL
terminal 70 nor SL terminal 72 (e.g. memory cell 150g) are shown in
FIGS. 32M through 32N, while the bias condition for memory cells
not sharing the same WL, SL, and BL terminals 70, 72, and 74
respectively as the selected memory cell 150a (e.g. memory cell
150h) is shown in FIG. 32O through 32P.
The floating body 24 potential of memory cells sharing the WL
terminal 70 as the selected memory cell will increase due to
capacitive coupling from WL terminal 70 by .DELTA.V.sub.FB. For
memory cells in state logic-0, the increase in the floating body 24
potential is not sustainable as the forward bias current of the p-n
diodes formed by floating body 24 and junction 16 will evacuate
holes from floating body 24. As a result, the floating body 24
potential will return to the initial state logic-0 equilibrium
potential. For memory cells in state logic-1, the floating body 24
potential will initially also increase by .DELTA.V.sub.FB, which
will result in holes being evacuated from floating body 24. After
the positive bias on the WL terminal 70 is removed, the floating
body 24 potential will decrease by .DELTA.V.sub.FB. If the initial
floating body 24 potential of state logic-1 is referred to as
V.sub.FB1, the floating body 24 potential after the write logic-0
operation will become V.sub.FB1-.DELTA.V.sub.FB. Therefore, the WL
potential needs to be optimized such that the decrease in floating
body potential of memory cells 50 in state logic-1 is not too
large. For example, the maximum floating body potential due to the
coupling from the WL potential cannot exceed V.sub.FB1/2.
As shown in FIGS. 32C through 32D, for unselected representative
memory cell 150b sharing the same WL terminal 70a and BL terminal
74a but not the same SL terminal 72 as the selected memory cell
150a, a negative bias is applied to the BL terminal while the SL
terminal is positively biased. The potential difference between the
BL and SL terminals (i.e. the emitter and collector terminals of
the bipolar device 30) is greater compared to the memory cells in
the holding mode. As a result, the forward bias current of the p-n
diode formed by floating body 24 and bit line region 16 is balanced
by higher base current of the bipolar device 30. As a result,
memory cell 150b will still be at holding mode. Thus, when memory
cell 150b is in state logic-1 it will maintain the charge in
floating body 24 because the intrinsic bipolar device 30 will
generate holes current to replenish the charge in floating body 24,
and when memory cell 150b is in state logic-0 the bipolar device 30
will remain off leaving the floating body 24 charge level a in
neutral state.
As shown in FIGS. 32E through 32F, for unselected representative
memory cell 150c sharing the same SL terminal 72b and BL terminal
74A but not the same WL terminal 70 as the selected memory cell
150a, the SL terminal 72 is now grounded with the BL terminal now
negatively biased. As a result, the p-n diode formed between
floating body 24 and bit line region 16 will be forward biased. For
memory cells in state logic-0, the increase in the floating body 24
potential will not change the initial state logic-0 as there is
initially no hole stored in the floating body 24. For memory cells
in state logic-1, the net effect is that the floating body 24
potential after write logic-0 operation will be reduced. Therefore,
the BL potential also needs to be optimized such that the decrease
in floating body potential of memory cells 50 in state logic-1 is
not too large. For example, the -V.sub.FB1/2 is applied to the BL
terminal 74. For memory cells in the logic-0 state, the bipolar
device 30 remains off leaving the cell in the logic-0 state.
As shown in FIGS. 32G through 32H, for unselected representative
memory cell 150d sharing the same WL terminal 70a and SL terminal
72b but not the same BL terminal 74 as the selected memory cell
150a, both the SL terminal 72 and BL terminal 74 are now grounded.
As a result, there is no potential difference between the emitter
and collector terminals of the n-p-n bipolar device 30 and
consequently memory cells 150d is no longer in holding mode.
However, because write operation is accomplished much faster (in
the order of nanoseconds) compared to the lifetime of the charge in
the floating body 24 (in the order of milliseconds), it should
cause little disruptions to the charge stored in the floating
body.
As shown in FIGS. 321 through 32J, for unselected representative
memory cell 150e sharing the same WL terminal 70a but not the same
SL terminal 72 nor BL terminal 74 as the selected memory cell 150a,
the SL terminal is positively biased. As a result, memory cell 150e
will still be at holding mode, where memory cells in state logic-1
will maintain the charge in floating body 24 because the intrinsic
bipolar device 30 will generate holes current to replenish the
charge in floating body 24, while memory cells in state logic-0
will remain in neutral state.
As shown in FIGS. 32K through 32L, for unselected representative
memory cell 150f sharing the same SL terminal 72b but not the same
WL terminal 70 nor BL terminal 74 as the selected memory cell 150a,
both the SL terminal 72 and BL terminal 74 are grounded. As a
result, there is no potential difference between the emitter and
collector terminals of the n-p-n bipolar device 30 and consequently
memory cells 150f is no longer in holding mode. However, because
write operation is accomplished much faster (in the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (in the order of milliseconds), it should cause little
disruptions to the charge stored in the floating body.
As shown in FIGS. 32M through 32N, for unselected representative
memory cell 150g sharing the same BL terminal 74a but not the same
WL terminal 70 nor SL terminal 72, a negative bias is applied to
the BL terminal while the SL terminal remains positively biased.
The potential difference between the BL and SL terminals (i.e. the
emitter and collector terminals of the bipolar device 30) is
greater compared to the memory cells in the holding mode. As a
result, the forward bias current of the p-n diode formed by
floating body 24 and bit line region 16 is balanced by higher base
current of the bipolar device 30. As a result, memory cell 150g
will still be at holding mode. Thus memory cells in state logic-1
will maintain the charge in floating body 24 because the intrinsic
bipolar device 30 will generate hole current to replenish the
charge in floating body 24, while memory cells in state logic-0
will remain in neutral state.
As shown in FIGS. 32O through 32P, for unselected representative
memory cell 150h not sharing WL, BL, and SL terminals 70, 74, and
72 respectively as the selected memory cell 150a, both the SL
terminal 72 will remain positively charged and the BL terminal will
remain grounded. As can be seen, these cells will be at holding
mode, where memory cells in state logic-1 will maintain the charge
in floating body 24 because the intrinsic bipolar device 30 will
generate holes current to replenish the charge in floating body 24,
while memory cells in state logic-0 will remain in neutral
state.
Several different types of a write logic-0 operation have been
described as examples illustrating the present invention. While
exemplary voltage levels have been given, the actual voltages used
in practice may vary substantially from embodiment to embodiment as
a matter of design choice and processing technology node used, and
are in no way limiting.
A write logic-1 operation can be performed on memory cell 150 by
means of impact ionization as described for example with reference
to Lin cited above, or by means of a band-to-band tunneling (GIDL)
mechanism, as described for example with reference to Yoshida cited
above.
Illustrated in FIG. 33, is an example of the bias condition of the
selected memory cell 150a in memory array 180 under a band-to-band
tunneling (GIDL) write logic-1 operation. The negative bias applied
to the WL terminal 70a and the positive bias applied to the BL
terminal 74a of the selected representative memory cell 150a result
in hole injection to the floating body 24 of the selected memory
cell 150 as discussed above with reference to Yoshida. The SL
terminal 72 and the substrate terminal 78 are grounded during the
write logic-1 operation.
As further illustrated in FIGS. 34A and 34B, in one particular
non-limiting embodiment, the following bias conditions are applied
to the selected memory cell 150a: a potential of about 0.0 volts is
applied to SL terminal 72b, a potential of about +1.2 volts is
applied to BL terminal 74a, a potential of about -1.2 volts is
applied to WL terminal 70a, and about 0.0 volts is applied to
substrate terminal 78 (not shown in FIG. 34B). This bias condition
bends the energy bands upward in the portion of bit line region 16
near the gate 60 in selected representative memory cell 150a
creating GIDL current on the bit line (electrons) while injecting
holes into the floating body 24 charging it up to a logic-1
level.
Also shown in FIG. 33, the following bias conditions are applied to
the unselected terminals: about +1.2 volts is applied to unselected
SL terminals 72a and 72c (not shown) through 72n+1, about 0.0 volts
is applied to unselected BL terminals 74b through 74p, a potential
of about 0.0 volts is applied to unselected WL terminals 70b
through 70n+1, and about 0.0 volts is applied to substrate
terminals 78a through 78n+1.
The unselected memory cells during write logic-1 operations are
shown in FIGS. 34C through 34O: The bias conditions for memory cell
150b sharing the same WL terminal 70a and BL terminal 74a but not
the same SL terminal 72 as the selected memory cell 150a are shown
in FIGS. 34C through 34D. The bias conditions for memory cell 150c
sharing the same SL terminal 72b and BL terminal 74a but not the
same WL terminal 70 as the selected memory cell 150a are shown in
FIGS. 34E through 34F. The bias conditions for memory cell 150d
sharing the same WL terminal 70a and SL terminal 72b but not the
same BL terminal 74 as the selected memory cell 150a are shown in
FIGS. 34G through 34H. FIGS. 341 through 34J show the bias
conditions for memory cell 150e sharing the same WL terminal 70a
but not the same SL terminal 72 nor BL terminal 74 as the selected
memory cell 150a. FIGS. 34K through 34L show the bias conditions
for memory cell 150f sharing the same SL terminal 72b but not the
same WL terminal 70 nor BL terminal 74 as the selected memory cell
150a. The bias conditions for memory cells sharing the same BL
terminal 74a as the selected memory cell 150a but not the same WL
terminal 70 nor SL terminal 72 (e.g. memory cell 150g) are shown in
FIGS. 34M through 34N, while the bias condition for memory cells
not sharing the WL, SL, and BL terminals 70, 72 and 74
respectively, as the selected memory cell 150a (e.g. memory cell
150h) are shown in FIG. 34O through 34P.
As shown in FIGS. 34C through 34D, for unselected representative
memory cell 150b sharing the same WL terminal 70a and BL terminal
74a but not the same SL terminal 72 as the selected memory cell
150a, both BL and SL terminals are positively biased. As a result,
there is no potential difference between the emitter and collector
terminals of the n-p-n bipolar device 30 and consequently memory
cell 150b is no longer in holding mode. However, because the write
operation is accomplished much faster (on the order of nanoseconds)
compared to the lifetime of the charge in the floating body 24 (on
the order of milliseconds), it should cause little disruption to
the charge stored in the floating body.
As shown in FIGS. 34E through 34F, for unselected representative
memory cell 150c sharing the same SL terminal 72b and BL terminal
74a but not the same WL terminal 70 as the selected memory cell
150a, the SL terminal 72 is now grounded with the BL terminal now
positively biased. As a result, memory cell 150c will be in a
holding mode, where memory cells in state logic-1 will maintain the
charge in floating body 24 because the intrinsic bipolar device 30
will generate hole current to replenish the charge in floating body
24 and memory cells in state logic-0 will remain in the neutral
state.
As shown in FIGS. 34G through 34H, for unselected representative
memory cell 150d sharing the same WL terminal 70a and SL terminal
72b but not the same BL terminal 74 as the selected memory cell
150a, both the SL terminal 72 and BL terminal 74 are now grounded.
As a result, there is no potential difference between the emitter
and collector terminals of the n-p-n bipolar device 30 and
consequently memory cell 150d is not in a holding mode. However,
because the write operation is accomplished much faster (on the
order of nanoseconds) compared to the lifetime of the charge in the
floating body 24 (on the order of milliseconds), it should cause
little disruption to the charge stored in the floating body.
As shown in FIGS. 341 through 34J, for unselected representative
memory cell 150e sharing the same WL terminal 70a but not the same
SL terminal 72 nor BL terminal 74 as the selected memory cell 150a,
the SL terminal remains positively biased. As a result, memory cell
150e will still be in a holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate holes current to
replenish the charge in floating body 24, and while memory cells in
state logic-0 will remain in a neutral state.
As shown in FIGS. 34K through 34L, for unselected representative
memory cell 150f sharing the same SL terminal 72b but not the same
WL terminal 70 nor BL terminal 74 as the selected memory cell 150a,
both the SL terminal 72 and BL terminal 74 are now grounded. As a
result, there is no potential difference between the emitter and
collector terminals of the n-p-n bipolar device 30 and consequently
memory cell 150f is no longer in a holding mode. However, because
the write operation is accomplished much faster (on the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (on the order of milliseconds), it should cause little
disruption to the charge stored in the floating body.
As shown in FIGS. 34M through 34N, for unselected representative
memory cell 150g sharing the same BL terminal 74a but not the same
WL terminal 70 nor SL terminal 72, a positive bias is applied to
the BL terminal and the SL terminal. As a result, there is no
potential difference between the emitter and collector terminals of
the n-p-n bipolar device 30 and consequently memory cell 150g is no
longer in a holding mode. However, because write operation is
accomplished much faster (on the order of nanoseconds) compared to
the lifetime of the charge in the floating body 24 (on the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
As shown in FIGS. 34O through 34P, for unselected representative
memory cell 150h not sharing WL, BL, and SL terminals 70, 74 and 72
respectively as the selected memory cell, the SL terminal 72n+1
will remain positively charged and the BL terminal 74b and the WL
terminal 70n are grounded. As can be seen, memory cell 150h will be
at holding mode, where memory cells in state logic-1 will maintain
the charge in floating body 24 because the intrinsic bipolar device
30 will generate hole current to replenish the charge in floating
body 24, while memory cells in state logic-0 will remain in the
neutral state.
Illustrated in FIG. 35, is an example of the bias condition of the
selected memory cell 150a in memory array 180 under an impact
ionization write logic-1 operation. The positive bias applied to
the WL terminal 70a and the positive bias applied to the BL
terminal 74a of the selected representative memory cell 150a
results in hole injection to the floating body 24 of the selected
memory cell 150 as discussed above with reference to Lin cited
above. The SL terminal 72b and the substrate terminals 78a through
78n+1 are grounded during the write logic-1 operation.
As further illustrated in FIG. 35, in one particular non-limiting
embodiment, the following bias conditions are applied to the
selected memory cell 150a: a potential of about 0.0 volts is
applied to SL terminal 72b, a potential of about +1.2 volts is
applied to BL terminal 74a, a potential of about +1.2 volts is
applied to WL terminal 70a, and about 0.0 volts is applied to
substrate terminals 78a through 78n+1. This bias condition causes
selected representative memory cell 150a to conduct current due to
the impact ionization mechanism discussed with reference to Lin
cited above. The combination of +1.2V on word line terminal and
+1.2V on bit line terminal 74a turns on the bipolar device 30 in
representative memory cell 150a regardless of its prior logic state
and generating sufficient hole charge in its floating body 24 to
place it in the logic-1 state.
Also shown in FIG. 35, the following bias conditions are applied to
the unselected terminals: about +1.2 volts is applied to unselected
SL terminals 72a and 72c (not shown) through 72n+1, about 0.0 volts
is applied to unselected BL terminals 74b through 74p, a potential
of about 0.0 volts is applied to unselected WL terminals 70b
through 70n+1, and about 0.0 volts is applied to substrate
terminals 78a through 78n+1.
For unselected representative memory cell 150b sharing the same WL
terminal 70a and BL terminal 74a but not the same SL terminal 72 as
the selected memory cell 150a, both BL and SL terminals are
positively biased. As a result, there is no potential difference
between the emitter and collector terminals of the n-p-n bipolar
device 30 and consequently memory cell 150b is no longer in a
holding mode. However, because the write operation is accomplished
much faster (in the order of nanoseconds) compared to the lifetime
of the charge in the floating body 24 (in the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
For unselected representative memory cell 150c sharing the same SL
terminal 72b and BL terminal 74a but not the same WL terminal 70 as
the selected memory cell 150a, the SL terminal 72b is now grounded
with the BL terminal now positively biased. As a result, memory
cell 150c will be in a holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate hole current to replenish
the charge in floating body 24, while memory cells in state logic-0
will remain in the neutral state.
For unselected representative memory cell 150d sharing the same WL
terminal 70a and SL terminal 72b but not the same BL terminal 74 as
the selected memory cell 150a, both the SL terminal 72 and BL
terminal 74 are now grounded. As a result, there is no potential
difference between the emitter and collector terminals of the n-p-n
bipolar device 30 and consequently memory cell 150d is not in a
holding mode. However, because the write operation is accomplished
much faster (in the order of nanoseconds) compared to the lifetime
of the charge in the floating body 24 (in the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
For unselected representative memory cell 150e sharing the same WL
terminal 70a but not the same SL terminal 72 nor BL terminal 74 as
the selected memory cell 150a, the SL terminal remains positively
biased. As a result, memory cell 150e will still be in a holding
mode, where memory cells in state logic-1 will maintain the charge
in floating body 24 because the intrinsic bipolar device 30 will
generate holes current to replenish the charge in floating body 24,
and while memory cells in state logic-0 will remain in a neutral
state. There is a possible write disturb issue with memory cell
150e in this case which will be discussed in more detail below in
conjunction with FIGS. 36A through 36B.
For unselected representative memory cell 150f sharing the same SL
terminal 72b but not the same WL terminal 70 nor BL terminal 74 as
the selected memory cell 150a, both the SL terminal 72 and BL
terminal 74 are now grounded. As a result, there is no potential
difference between the emitter and collector terminals of the n-p-n
bipolar device 30 and consequently memory cell 150f is no longer in
a holding mode. However, because the write operation is
accomplished much faster (in the order of nanoseconds) compared to
the lifetime of the charge in the floating body 24 (in the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
For unselected representative memory cell 150g sharing the same BL
terminal 74a but not the same WL terminal 70 nor SL terminal 72, a
positive bias is applied to the BL terminal 74a and the SL terminal
72n+1. As a result, there is no potential difference between the
emitter and collector terminals of the n-p-n bipolar device 30 and
consequently memory cell 150g is no longer in a holding mode.
However, because the write operation is accomplished much faster
(in the order of nanoseconds) compared to the lifetime of the
charge in the floating body 24 (in the order of milliseconds), it
should cause little disruption to the charge stored in the floating
body.
For unselected representative memory cell 150h not sharing WL, BL,
and SL terminals 70, 74 and 72 respectively as the selected memory
cell 150a, the SL terminal 72n+1 will remain positively charged and
the BL terminal 74b and the WL terminal 70n are grounded. As can be
seen, memory cell 150h will be at holding mode, where memory cells
in state logic-1 will maintain the charge in floating body 24
because the intrinsic bipolar device 30 will generate holes current
to replenish the charge in floating body 24; while memory cells in
state logic-0 will remain in neutral state.
FIGS. 36A and 36B illustrate the bias conditions of representative
memory cell 150e under the bias conditions shown in FIG. 35. Memory
cell 150e is coupled to word line terminal 70a biased at +1.2V, bit
line terminal 74b biased at 0.0V, and source line terminal 72a
biased at +1.2V. The concern is that the floating body region 24 of
memory cell 150e boosted by capacitive coupling from word line 70a
while having 1.2 volts of bias across it--albeit of the opposite
voltage potential of selected representative memory cell 150a. If
bipolar device 30 were to turn on under these conditions, a write
disturb condition (writing an unwanted logic-1 in an unselected
memory cell) would occur with a logic-1 being written into
unselected memory cell 150e.
One solution to the write disturb in representative memory cell
150e is to design memory cell 150 such that the impact ionization
is less efficient at generating charge carriers when the source
line terminal 72 is positively biased than it is in the case when
the bit line terminal 74 is positively biased using techniques
known in the art. This creates enough current to place
representative memory cell 150e in a holding mode while generating
a larger current sufficient for writing a logic-1 in memory cell
150a.
Alternatively, a different set of bias conditions may be used as
illustrated in FIG. 37 which shows another example of writing
logic-1 into selected memory cell 150a in memory array 180 using
impact ionization. As in FIG. 35, the positive bias applied to the
WL terminal 70a and the positive bias applied to the BL terminal
74a of the selected representative memory cell 150a results in hole
injection to the floating body 24 of the selected memory cell 150
as discussed above with reference to Lin cited above. The SL
terminal 72b and the substrate terminals 78a through 78n+1 are
grounded during the write logic-1 operation. The difference in this
write logic-1 operation are the bias conditions of the unselected
bit lines 74b through 74p and the unselected source lines 72a and
72c (not shown) through 72n+1.
As further illustrated in FIG. 37, in one particular non-limiting
embodiment, the following bias conditions are applied to the
selected memory cell 150a: a potential of about 0.0 volts is
applied to SL terminal 72b, a potential of about +1.2 volts is
applied to BL terminal 74a, a potential of about +1.2 volts is
applied to WL terminal 70a, and about 0.0 volts is applied to
substrate terminals 78a through 78n+1. This bias condition causes
selected representative memory cell 150a to conduct current due to
the impact ionization mechanism discussed with reference to Lin
cited above. The combination of +1.2V on word line terminal and
+1.2V on bit line terminal 74a turns on the bipolar device 30 in
representative memory cell 150a regardless of its prior logic state
and generating sufficient hole charge in its floating body 24 to
place it in the logic-1 state.
Also shown in FIG. 37, the following bias conditions are applied to
the unselected terminals: about +0.6 volts is applied to unselected
SL terminals 72a and 72c (not shown) through 72n+1, about +0.6
volts is applied to unselected BL terminals 74b through 74p, a
potential of about 0.0 volts is applied to unselected WL terminals
70b through 70n+1, and about 0.0 volts is applied to substrate
terminals 78a through 78n+1.
For unselected representative memory cell 150b sharing the same WL
terminal 70a and BL terminal 74a but not the same SL terminal 72 as
the selected memory cell 150a, both BL and SL terminals are
positively biased with a larger bias applied to the BL than the SL.
As a result, bipolar device 30 is on and memory cell 150b is in a
holding mode, where memory cells in state logic-1 will maintain the
charge in floating body 24 because the intrinsic bipolar device 30
will generate hole current to replenish the charge in floating body
24, while memory cells in state logic-0 will remain in the neutral
state.
For unselected representative memory cell 150c sharing the same SL
terminal 72b and BL terminal 74a but not the same WL terminal 70 as
the selected memory cell 150a, the SL terminal 72b is now grounded
with the BL terminal now positively biased. As a result, memory
cell 150c will be in a holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate hole current to replenish
the charge in floating body 24, while memory cells in state logic-0
will remain in the neutral state.
For unselected representative memory cell 150d sharing the same WL
terminal 70a and SL terminal 72b but not the same BL terminal 74 as
the selected memory cell 150a, the SL terminal 72b is now grounded
and the BL terminal 74b has a slight positive bias. As a result,
memory cell 150d will be in a holding mode, where memory cells in
state logic-1 will maintain the charge in floating body 24 because
the intrinsic bipolar device 30 will generate hole current to
replenish the charge in floating body 24, while memory cells in
state logic-0 will remain in the neutral state.
For unselected representative memory cell 150e sharing the same WL
terminal 70a but not the same SL terminal 72 nor BL terminal 74 as
the selected memory cell 150a, the SL terminal 72a and the BL
terminal 74b both have a slight positive bias. As a result, there
is no potential difference between the emitter and collector
terminals of the n-p-n bipolar device 30 and consequently memory
cell 150e is no longer in a holding mode. However, because the
write operation is accomplished much faster (in the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (in the order of milliseconds), it should cause little
disruption to the charge stored in the floating body. This also
eliminates the potential write disturb condition for representative
memory cell 150e present with the bias conditions of FIGS. 35, 36A
and 36B.
For unselected representative memory cell 150f sharing the same SL
terminal 72b but not the same WL terminal 70 nor BL terminal 74 as
the selected memory cell 150a, the SL terminal 72b is grounded and
BL terminal 74b has a small positive bias. As a result, memory cell
150f will be in a holding mode, where memory cells in state logic-1
will maintain the charge in floating body 24 because the intrinsic
bipolar device 30 will generate hole current to replenish the
charge in floating body 24, while memory cells in state logic-0
will remain in the neutral state.
For unselected representative memory cell 150g sharing the same BL
terminal 74a but not the same WL terminal 70 nor SL terminal 72, a
positive bias is applied to the BL terminal 74a and a smaller
positive bias is applied to SL terminal 72n+1. As a result, memory
cell 150g will be in a holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 30 will generate hole current to replenish
the charge in floating body 24, while memory cells in state logic-0
will remain in the neutral state.
For unselected representative memory cell 150h not sharing WL, BL,
and SL terminals 70, 74 and 72 respectively as the selected memory
cell 150a, the SL terminal 72n+1 and the BL terminal 74b will have
a slight positive bias while the WL terminal 70n is grounded. As a
result, there is no potential difference between the emitter and
collector terminals of the n-p-n bipolar device 30 and consequently
memory cell 150e is no longer in a holding mode. However, because
the write operation is accomplished much faster (in the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (in the order of milliseconds), it should cause little
disruption to the charge stored in the floating body.
Different structures and methods of operation have been discussed
with respect to exemplary memory array 180 comprised of a plurality
of memory cells 150. Many other embodiments are possible within the
scope of the invention. For example, regions of the first
conductivity type may be changed from p-type to n-type and regions
of the second conductivity type may be changed from n-type to
p-type combined with a reversal of the polarities of the bias
voltages for various operations. The bias levels themselves are
exemplary only and will vary from embodiment to embodiment as a
matter of design choice. Memory array 180 may be altered so that
the outside rows share a source line 72 with the adjacent row and
have a dedicated word line 70. Many other embodiments will readily
suggest themselves to persons skilled in the art, thus the
invention is not to be limited in any way except by the appended
claims.
It is noteworthy that memory cell 550 constructed using either of
the fin structures 52 and 52A described in conjunction with FIGS.
20 through 22B can be used to replace memory cell 150 in memory
array 180 with shared word lines with or without shared source
lines and will function in a manner similar to that described for
memory cell 150. Many other modifications may also made to array
150. For example, the first and second conductivity types may be
reversed in either memory cell 150 or memory cell 550 with reversal
of the relative polarities of the applied voltages. All of the
voltage levels described are exemplary only and will very from
embodiment to embodiment as a matter of design choice. Thus the
invention is not to be limited in any way.
FIG. 38A shows another embodiment of the memory device 150, in
which adjacent regions 16, which are separated by insulating layer
28, share a common connection to BL terminal 74 through contact 64.
By sharing a common connection to the BL terminal 74, a more
compact memory cell can be obtained as only one contact is required
for each two memory cells 150.
Another embodiment of memory cell 150 is shown in FIG. 38B, where
bit line region 16 and contact 64 are now shared between two
adjacent memory cells 150. Isolation of the adjacent floating body
24 regions of a first conductivity type is achieved through both
insulating region 33 and bit line region 16 of a second
conductivity type.
FIGS. 39A through 39O describe a method of manufacturing memory
cell 150 as shown in FIG. 38B created using, in part, a replacement
insulator technique like that described in S_Kim and Oh discussed
above to create insulating region 33.
A method of manufacturing memory cell 150 as shown in FIG. 38B will
be described with reference to FIGS. 39A through 39AA. These 27
figures are arranged in groups of three related views, with the
first figure of each group being a top view, the second figure of
each group being a vertical cross section of the top view in the
first figure of the group designated I-I', and the third figure of
each group being a horizontal cross section of the top view in the
first figure of the group designated II-II'. Thus FIGS. 39A, 39D,
39G, 39J, 39M, 39P, 39S, 39V, and 39Y are a series of top views of
the memory cell 50 at various stages in the manufacturing process,
FIGS. 39B, 39E, 39H, 39K, 39N, 39Q, 39T, 39W, and 39Z are their
respective vertical cross sections labeled I-I', and FIGS. 39C,
39F, 39I, 39L, 39O, 39R, 39U, 39X, and 39AA are their respective
horizontal cross sections labeled II-II'. Identical reference
numbers from earlier drawing figures appearing in FIGS. 39A through
39AA represent similar, identical, or analogous structures as
previously described in conjunction with the earlier drawing
figures. Here "vertical" means running up and down the page in the
top view diagram and "horizontal" means running left and right on
the page in the top view diagram. In the physical embodiment of
memory cell 50, both cross sections are "horizontal" with respect
to the surface of the semiconductor device.
As illustrated in FIGS. 39A through 39C, a thin conductive region
202 (e.g. 300 A in an exemplary 130 nm process, though this will
vary with embodiments in different process technologies and
geometries) is grown on the surface of substrate 12. The conductive
region 202 is comprised of a different material from the materials
of the substrate region 12 so that it may be selectively etched
away later without simultaneous unwanted etching of substrate 12.
For example, the conductive region 202 could be made of silicon
germanium (SiGe) material, while substrate 12 could be made of
silicon.
As illustrated in FIGS. 39D through 39F, a lithography process is
then performed to pattern the conductive region 202. Subsequently,
layer 202 is etched, followed by another conductive region 204
growth. As an example, the thickness of region 204 is about 500 A
in an exemplary 130 nm process. Region 204 may comprise of the same
materials forming substrate 12, for example silicon. A
planarization step can then be performed to ensure a planar
surface. The resulting structure can be seen in FIGS. 39D through
39F.
As illustrated in FIGS. 39G through 39H, a trench formation process
is then performed, which follows a similar sequence of steps as
shown in FIGS. 2A through 2I, i.e. formation of silicon oxide 220,
polysilicon 222, and silicon nitride 224 layers, followed by
lithography patterning and etch processes. Trench 216 is etched
such that the trench depth is deeper than that of trench 208. For
example, the trench 208 depth is about 1200 A, while the trench 216
depth is about 2000 A in an exemplary 130 nm process. The resulting
structures are shown in FIGS. 39G through 39I.
As illustrated in FIGS. 39J through 39L, this is then followed by
silicon oxidation step, which will grow silicon oxide films in
trench 208 and trench 216. For example, about 4000 A silicon oxide
can be grown in an exemplary 130 nm process. A chemical mechanical
polishing step can then be performed to polish the resulting
silicon oxide films so that the silicon oxide layer is flat
relative to the silicon surface. A silicon dry etching step can
then be performed so that the remaining silicon oxide layer height
is about 300 A from the silicon surface in an exemplary 130 nm
process. The silicon nitride layer 224 and the polysilicon layer
222 can then be removed, followed by a wet etch process to remove
silicon oxide layer 220 (and a portion of the silicon oxide films
formed in the area of former trench 208 and trench 216). FIGS. 39J
through 39L show the insulating layers 26 and 28 formed following
these steps.
As illustrated in FIGS. 39M through 39O, an oxide etch is then
performed to recess the oxide regions 26 and 28 (for example by
about 1000 A) to expose the conductive region 202. A wet etch
process is then performed to selectively remove region 202 leaving
an gap 203 under an overhanging portion of region 204. The
resulting structures following these steps are shown in FIGS. 39M
through 39O.
As illustrated in FIGS. 39P through 39R, the resulting gap region
203 is then oxidized to form a buried oxide region 33. Recessing
insulating region 26 down to the surface of substrate 12 allows
access for the etch of region 202 to form gap 203 and then
subsequent oxide growth in gap 203 to form buried oxide region 33.
The overhanging portion of region 204 constrains the oxide growth
in gap space 203 to keep the buried oxide region 33 from growing to
the surface. The resulting structures are shown in FIGS. 39P
through 39R.
As illustrated in FIGS. 39S through 39U, an oxide deposition of
about 1000 A is then performed followed by a planarization process.
This is then followed by an ion implantation step to form the
buried well region 22. The ion implantation energy is optimized
such that the buried layer region 22 is formed shallower than the
bottom of the insulating layer 26. As a result, the insulating
layer 26 isolates buried layer region 22 between adjacent cells. On
the other hand, the buried layer region 22 is formed such that
insulating layers 28 and 33 do not isolate buried layer region 22,
allowing buried layer region 22 to be continuous in the direction
of II-II' cross section line. Following these steps, the resulting
structures are shown in FIGS. 39S through 39U.
As illustrated in FIGS. 39V through 39X, a silicon oxide layer (or
high-dielectric materials) 62 is then formed on the silicon surface
(e.g. about 100 A in an exemplary 130 nm process), followed by a
polysilicon (or metal) gate 60 deposition (e.g. about 500 A in an
exemplary 130 nm process). A lithography step is then performed to
form the pattern for the gate and word line, followed by etching of
the polysilicon and silicon oxide layers where they are not waned.
The resulting structure is shown in FIGS. 39V-39X.
As illustrated in FIGS. 39Y through 39AA, another ion implantation
step is then performed to form the bit line region 16 of a second
conductivity type (e.g. n-type conductivity). The gate 60 and the
insulating layers 26 and 28 serve as masking layer for the
implantation process such that regions of second conductivity are
not formed outside bit line region 16. This is then followed by
backend process to form contact and metal layers.
FIGS. 40A through 40F illustrate an embodiment of a Gateless Half
Transistor memory cell. Memory cell 250 according to the present
invention eliminates the gate terminal present in earlier memory
cells such as memory cell 50 allowing a more compact layout since
some design rules like gate-to-contact-spacing no longer affect the
minimum cell size.
Present in FIGS. 40A through 40F are substrate 12 of the first
conductivity type, buried layer 22 of the second conductivity type,
bit line region 16 of the second conductivity type, region of the
second conductivity type 20, region of the first conductivity type
21, buried layer region 22, floating body 24 of the first
conductivity type, insulating regions 26 and 28, source line
terminal 72, and substrate terminal 78 all of which perform
substantially similar functions in memory cell 250 as in previously
discussed embodiment memory cell 50. The primary difference between
memory cell 250 and memory cell 50 previously discussed is the
absence of gate 60 and gate insulator 62. As in other embodiments,
there is no contact to the buried layer 22 at the semiconductor
surface 14 inside the boundary of memory cell 250.
The manufacturing of memory cell 250 is substantially similar to
the manufacturing of memory cell 50 described in conjunction with
FIGS. 2A through 2U, except that instead of a lithographic step for
forming gate 60, a different lithographic step is needed to pattern
bit line region 16 for implantation or diffusion.
FIG. 40A illustrates a top view of memory cell 250 with several
near neighbors.
FIG. 40B illustrates a top view a single memory cell 250 with
vertical cut line I-I' and horizontal cut line II-II' for the cross
sections illustrated in FIGS. 40C and 40D respectively.
FIG. 40E shows how memory cell 250 may have its buried layer 22
coupled to source line terminal 72 through region 20 of the second
conductivity type and its substrate 12 coupled to substrate
terminal 78 through region of first conductivity type 21.
FIG. 40F shows exemplary memory array 280 which will be used in
subsequent drawing figures to illustrate the various operations
that may be performed on memory cell 250 when arranged in an array
to create a memory device. Array 280 comprises in part
representative memory cells 250a, 250b, 250c and 250d. In
operations where a single memory cell is selected, representative
memory cell 250a will represent the selected cell while the other
representative memory cells 250b, 250c and 250d will represent the
various cases of unselected memory cells sharing a row, sharing a
column, or sharing neither a row or a column respectively with
selected representative memory cell 250a. Similarly in the case of
operations performed on a single row or column, representative
memory cell 250a will always be on the selected row or column.
While the drawing figures show the first conductivity type as
p-type and the second conductivity type as n-type, as with previous
embodiments the conductivity types may be reversed with the first
conductivity type becoming n-type and the second conductivity type
becoming p-type as a matter of design choice in any particular
embodiment.
The memory cell states are represented by the charge in the
floating body 24, which modulates the intrinsic n-p-n bipolar
device 230 formed by buried well region 22, floating body 24, and
BL bit line region 16. If cell 250 has holes stored in the body
region 24, then the memory cell will have a higher bipolar current
(e.g. current flowing from BL to SL terminals during read
operation) compared to if cell 250 does not store holes in body
region 24.
The positive charge stored in the body region 24 will decrease over
time due to the p-n diode leakage formed by floating body 24 and
bit line region 16 and buried layer 22 and due to charge
recombination. A unique capability of the invention is the ability
to perform the holding operation in parallel to all memory cells of
the array.
An entire array holding operation is illustrated in FIG. 41A while
a single row holding operation is illustrated in FIG. 41B. The
holding operation can be performed in a manner similar to the
holding operation for memory cell 50 by applying a positive bias to
the back bias terminal (i.e. SL terminal 72) while grounding
terminal 74 and substrate terminal 78. If floating body 24 is
positively charged (i.e. in a state logic-1), the n-p-n bipolar
transistor 230 formed by BL bit line region 16, floating body 24,
and buried well region 22 will be turned on.
A fraction of the bipolar transistor current will then flow into
floating region 24 (usually referred to as the base current) and
maintain the state logic-1 data. The efficiency of the holding
operation can be enhanced by designing the bipolar device formed by
buried well region 22, floating region 24, and bit line region 16
to be a low-gain bipolar device, where the bipolar gain is defined
as the ratio of the collector current flowing out of SL terminal 72
to the base current flowing into the floating region 24.
For memory cells in state logic-0 data, the bipolar device will not
be turned on, and consequently no base hole current will flow into
floating region 24. Therefore, memory cells in state logic-0 will
remain in state logic-0.
A periodic pulse of positive voltage can be applied to the SL
terminal 72 as opposed to applying a constant positive bias to
reduce the power consumption of the memory cell 250.
An example of the bias condition for the holding operation is
hereby provided: zero voltage is applied to BL terminal 74, a
positive voltage is applied to SL terminal 72, and zero voltage is
applied to the substrate terminal 78. In one particular
non-limiting embodiment, about +1.2 volts is applied to terminal
72, about 0.0 volts is applied to terminal 74, and about 0.0 volts
is applied to terminal 78. However, these voltage levels may vary
from embodiment to embodiment as a matter of design choice.
In the entire array holding operation of FIG. 41A, all of the
source line terminals 72a through 72n are biased at +1.2V, all of
the bit lines 74a through 74p are biased to 0.0V, and all of the
source terminals 78a through 78n are biased to 0.0V. This places
all of the cells in memory array 280 in the hold state.
In the single row hold operation of FIG. 41B, selected source line
terminal 72a is biased at +1.2V while the unselected source line
terminals 72b (not shown) through 72n are biased at 0.0V, all of
the bit lines 74a through 74p are biased to 0.0V, and all of the
source terminals 78a through 78n are biased to 0.0V. This places
all of the selected cells in memory array 280 in the hold
state.
A single memory cell read operation is illustrated in FIGS. 42 and
42A through 42H. The read operation for memory cell 250 can be
performed by sensing the current of the bipolar device 230 by
applying the following bias condition: a positive voltage is
applied to the selected BL terminal 74, zero voltage is applied to
the selected SL terminal 72, and zero voltage is applied to the
substrate terminal 78. The positive voltage applied to the selected
BL terminal is less than or equal to the positive voltage applied
to the SL terminal during holding operation. The unselected BL
terminals will remain at zero voltage and the unselected SL
terminals will remain at positive voltage.
FIG. 42 shows the bias condition for the selected memory cell 250a
and unselected memory cells 250b, 250c, and 250d in memory array
280. In this particular non-limiting embodiment, about 0.0 volts is
applied to the selected SL terminal 72a while about 0.0V is applied
to the unselected source line terminals 72b (not shown) through
72n, about +1.2 volts is applied to the selected BL terminal 74a
while 0.0V is applied to the unselected bit line terminals 74b
through 74p, and about 0.0 volts is applied to substrate terminals
78a through 78n. These voltage levels are exemplary only and may
vary from embodiment to embodiment.
In FIGS. 42A and 42B, the bias conditions for selected
representative memory cell 250a are shown. In this particular
non-limiting embodiment, about 0.0 volts is applied to the selected
SL terminal 72a, about +1.2 volts is applied to the selected BL
terminal 74a, and about 0.0 volts is applied to substrate terminal
78 (not shown). This causes current to flow through intrinsic
bipolar device 230 if the floating body is positively charged and
no current to flow if the floating body is discharged since the
bipolar device 230 is off.
The unselected memory cells during read operations are shown in
FIGS. 42C through 42H. The bias conditions for memory cells sharing
the same row (e.g. memory cell 250b) are shown in FIGS. 42C and
42D. The bias conditions for memory cells sharing the same column
(e.g. memory cell 250c) as the selected memory cell 250a are shown
in FIGS. 42E and 42F. The bias conditions for memory cells sharing
neither the same row nor the same column as the selected memory
cell 250a (e.g. memory cell 250d) are shown in FIG. 42G-42H.
As illustrated in FIGS. 42C and 42D, for memory cell 250b sharing
the same row as the selected memory cell 250a, the SL terminal 72a
and the BL terminal 74p are both biased to 0.0V and consequently
these cells will not be at the holding mode. However, because read
operation is accomplished much faster (in the order of nanoseconds)
compared to the lifetime of the charge in the floating body 24 (in
the order of milliseconds), it should cause little disruption to
the charge stored in the floating body.
As illustrated in FIGS. 42E and 42F, for memory cell 250c sharing
the same column as the selected memory cell 250a, a positive
voltage is applied to the BL terminal 74a and SL terminal 72n. No
base current will flow into the floating body 24 because there is
no potential difference between SL terminal 72 and BL terminal 74
(i.e. the emitter and collector terminals of the n-p-n bipolar
device 230). However, because read operation is accomplished much
faster (in the order of nanoseconds) compared to the lifetime of
the charge in the floating body 24 (in the order of milliseconds),
it should cause little disruption to the charge stored in the
floating body.
As illustrated in FIGS. 42G and 42H, for memory cell 250d sharing
neither the same row nor the same column as the selected memory
cell 250a, both the SL terminal 72n will remain positively charged
and the BL terminal 74p remain grounded. Representative memory cell
250d will be in the holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 because the
intrinsic bipolar device 230 will generate hole current to
replenish the charge in floating body 24, while memory cells in
state logic-0 will remain in neutral state.
The various voltage bias levels above are exemplary only. They will
vary from embodiment to embodiment as a function of both design
choice and the process technology used.
FIG. 43 illustrates a single row write logic-0 operation while
FIGS. 44A and 44B illustrate the biasing conditions and operation
of unselected representative memory cell 250c. In FIG. 43 the
selected row SL terminal 72a is biased negatively at about -0.5V
while the unselected row SL terminals 72b (not shown) through 72n
are biased at about 0.0V, all the BL terminals 74a through 74p are
biased at 0.0V, and all the substrate terminals 78a through 78n are
biased at 0.0V. This causes the selected cells 250 like
representative memory cells 250a and 250b to have their bipolar
devices turn on due to forward bias on the floating body 24 to
buried layer 22 junction evacuating the holes from the floating
body 24.
FIGS. 44A and 44B show the operation of unselected representative
memory cell 250c which in this case is representative of all the
memory cells 250 in memory array 280 not on the selected row.
Memory cell 250c has its SL terminal 72n at +1.2V and its BL
terminal 74a at 0.0V which corresponds to the holding operation
described above in conjunction with FIGS. 41A and 41B.
A write logic-0 operation can also be performed on a column basis
by applying a negative bias to the BL terminal 74 as opposed to the
SL terminal 72. The SL terminal 72 will be zero or positively
biased, while zero voltage is applied to the substrate terminal 78.
Under these conditions, all memory cells sharing the same BL
terminal 74 will be written into state logic-0 and all the other
cells will be in a holding operation.
The various voltage bias levels above are exemplary only. They will
vary from embodiment to embodiment as a function of both design
choice and the process technology used.
A write logic-1 operation can be performed on memory cell 250
through impact ionization as described for example with reference
to Lin above.
An example of the bias condition of the selected memory cell 250a
under impact ionization write logic-1 operation is illustrated in
FIG. 45 and FIGS. 46A through 46B. A positive bias is applied to
the BL terminal 74, while zero voltage is applied to the selected
SL terminal 72 and substrate terminal 78. The positive bias applied
to the BL terminal 74 is greater than the positive voltage applied
to the SL terminal 72 during holding operation. The positive bias
applied to the BL terminal is large enough to turn on bipolar
device 230 regardless of the initial state of the data in selected
memory cell 250a. This results in a base hole current to the
floating body 24 of the selected memory cell 250a charging it up to
a logic-1 state.
In one particular non-limiting embodiment, the following bias
conditions are applied to the selected memory cell 250a: a
potential of about 0.0 volts is applied to selected SL terminal
72a, a potential of about +2.0 volts is applied to selected BL
terminal 74a, and about 0.0 volts is applied to substrate terminals
78a through 78n. The following bias conditions are applied to the
unselected terminals: about +1.2 volts is applied to SL terminals
72b (not shown) through 72n, and about 0.0 volts is applied to BL
terminals 74b through 74p. FIG. 45 shows the bias condition for the
selected and unselected memory cells in memory array 280. The
various voltage bias levels above are exemplary only. They will
vary from embodiment to embodiment as a function of both design
choice and the process technology used.
The unselected memory cells during write logic-1 operations are
shown in FIGS. 46C through 46H. The bias conditions for memory
cells sharing the same row (e.g. memory cell 250b) are shown in
FIGS. 46C through 46D, the bias conditions for memory cells sharing
the same column as the selected memory cell 250a (e.g. memory cell
250c) are shown in FIGS. 46E through 46F, and the bias conditions
for memory cells 250 not sharing the same row nor the same column
as the selected memory cell 250a (e.g. memory cell 250d) are shown
in FIGS. 46G through 46H.
As shown in FIGS. 46C and 46D, for representative memory cell 250b
sharing the same row as the selected memory cell 250a, SL terminal
72a and BL terminal 74p are be grounded. Bipolar device 230 will be
off and the memory cell 250b will not be at the holding mode.
However, because write operation is accomplished much faster (in
the order of nanoseconds) compared to the lifetime of the charge in
the floating body 24 (in the order of milliseconds), it should
cause little disruption to the charge stored in the floating
body.
As shown in FIGS. 46E and 46F, for representative memory cell 250c
sharing the same column as the selected memory cell 250a, a greater
positive voltage is applied to the BL terminal 74a and a lesser
positive voltage is applied to SL terminal 72n. Less base current
will flow into the floating body 24 than in selected memory cell
250a because of the lower potential difference between SL terminal
72 and BL terminal 74 (i.e. the emitter and collector terminals of
the n-p-n bipolar device 230). However, because write operation is
accomplished much faster (in the order of nanoseconds) compared to
the lifetime of the charge in the floating body 24 (in the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
As shown in FIGS. 46G and 46H, for representative memory cell 250d
sharing neither the same column nor the same row as the selected
memory cell 250a, the SL terminal 72 is positively charged and the
BL terminal is grounded. Representative memory cell 250d will be at
holding mode, where memory cells in state logic-1 will maintain the
charge in floating body 24 because the intrinsic bipolar device 230
will generate hole current to replenish the charge in floating body
24 and where memory cells in state logic-0 will remain in neutral
state.
The various voltage bias levels above are exemplary only. They will
vary from embodiment to embodiment as a function of both design
choice and the process technology used. Also, the first
conductivity type may be changed from p-type to n-type and the
second conductivity type may be changed from n-type to p-type, and
the polarities of the applied biases may be reversed. Thus the
invention is not to be limited in any way except by the appended
claims.
A vertical stack of alternating conductive regions of first
conductivity type and second conductivity type has been described
in J_Kim discussed above, where a gate is overlaid surrounding the
body region 24 on two sides. By removing the gates, a more compact
memory cell than that reported in J_Kim can be obtained as in
memory cell 350 discussed below.
FIGS. 47A through 47F illustrate another embodiment of a Gateless
Half Transistor memory cell. By allowing the bit line region 16 to
completely cover the floating body region 24 in memory cell 350,
some design rules like minimum-diffusion-to-insulator-spacing (the
space from 16 to 26 in memory cell 250) no longer affects the cell
size. Present in FIGS. 47A through 40F are substrate 12 of the
first conductivity type, buried layer 22 of the second conductivity
type, bit line region 16 of the second conductivity type, region of
the second conductivity type 20, region of the first conductivity
type 21, floating body 24 of the first conductivity type, buried
layer region 22, insulating regions 26 and 28, source line terminal
72, and substrate terminal 78 all of which perform substantially
similar functions in memory cell 350 as in previously discussed
embodiment memory cell 250. The primary difference between memory
cell 350 and memory cell 250 previously discussed is that bit line
region 16 completely covers a (now smaller) floating body region 24
allowing for a more compact memory cell. As in other embodiments,
there is no contact to the buried layer 22 at the semiconductor
surface 14 inside the boundary of memory cell 350.
The manufacturing of memory cell 350 is substantially similar to
the manufacturing of memory cell 50 described in conjunction with
FIGS. 2A through 2U and memory cell 250 described in conjunction
with FIGS. 40A through 40F above, except that bit line region 16
may be formed by an implantation process formed on the material
making up substrate 12 according to any of implantation processes
known and typically used in the art. Alternatively, solid state
diffusion or epitaxial growth process may also be used to form bit
line region 16.
FIG. 47A illustrates a top view of memory cell 350 with several
near neighbors.
FIG. 47B illustrates a top view a single memory cell 350 with
vertical cut line I-I' and horizontal cut line II-II' for the cross
sections illustrated in FIGS. 47C and 47D respectively.
FIG. 40E shows how memory cell 350 may have its buried layer 22
coupled to source line terminal 72 through region 20 of the second
conductivity type and its substrate 12 coupled to substrate
terminal 78 through region 21 of the first conductivity type.
FIG. 47F shows exemplary memory array 380 comprising multiple
memory cells 350 when arranged in an array to create a memory
device. The circuit operation of memory cell 350 is substantially
identical to that of memory cell 250 and will not be discussed
further.
While the drawing figures show the first conductivity type as
p-type and the second conductivity type as n-type, as with previous
embodiments the conductivity types may be reversed with the first
conductivity type becoming n-type and the second conductivity type
becoming p-type as a matter of design choice in any particular
embodiment.
An alternate method of operating memory cells 50, 150, and 450,
which utilizes the silicon controlled rectifier (SCR) principle
discussed above with reference to Widjaja, is now described.
As shown in FIG. 48, inherent in memory cells 50, 150 and 450 is a
P1-N2-P3-N4 silicon controlled rectifier (SCR) device formed by two
interconnected bipolar devices 32 and 34, with substrate 78
functioning as the P1 region, buried layer 22 functioning as the N2
region, body region 24 functioning as the P3 region and bit line
region 16 functioning as the N4 region. In this example, the
substrate terminal 78 functions as the anode and terminal 74
functions as the cathode, while body region 24 functions as a
p-base to turn on the SCR device. If body region 24 is positively
charged, the silicon controlled rectifier (SCR) device formed by
the substrate, buried well, floating body, and the BL junction will
be turned on and if body region 24 is neutral, the SCR device will
be turned off.
The holding operation can be performed by applying the following
bias: zero voltage is applied to BL terminal 74, zero or negative
voltage is applied to WL terminal 70, and a positive voltage is
applied to the substrate terminal 78, while leaving SL terminal 72
floating. Under these conditions, if memory cell 50 is in
memory/data state logic-1 with positive voltage in floating body
24, the SCR device of memory cell 50 is turned on, thereby
maintaining the state logic-1 data. Memory cells in state logic-0
will remain in blocking mode, since the voltage in floating body 24
is not substantially positive and therefore floating body 24 does
not turn on the SCR device. Accordingly, current does not flow
through the SCR device and these cells maintain the state logic-0
data. Those memory cells 50 that are commonly connected to
substrate terminal 78 and which have a positive voltage in body
region 24 will be refreshed with a logic-1 data state, while those
memory cells 50 that are commonly connected to the substrate
terminal 78 and which do not have a positive voltage in body region
24 will remain in blocking mode, since their SCR device will not be
turned on, and therefore memory state logic-0 will be maintained in
those cells. In this way, all memory cells 50 commonly connected to
the substrate terminal will be maintained/refreshed to accurately
hold their data states. This process occurs automatically, upon
application of voltage to the substrate terminal 78, in a parallel,
non-algorithmic, efficient process. In one particular non-limiting
embodiment, a voltage of about 0.0 volts is applied to terminal 74,
a voltage of about -1.0 volts is applied to terminal 70, and about
+0.8 volts is applied to terminal 78. However, these voltage levels
may vary, while maintaining the relative relationships there
between.
As illustrated in FIG. 49, a read operation can be performed by
applying a positive voltage to the substrate terminal 78, a
positive voltage (lower than the positive voltage applied to the
substrate terminal 78) to BL terminal 74, a positive voltage to WL
terminal 70, while leaving SL terminal 72 floating. If cell 50a is
in a state logic-1 having holes in the body region 24, the silicon
controlled rectifier (SCR) device formed by the substrate, buried
well, floating body, and the BL junction will be turned on and a
higher cell current (flowing from the substrate terminal 78 to the
BL terminal 74) is observed compared to when cell 50 is in a state
logic-0 having no holes in body region 24. A positive voltage is
applied to WL terminal 70a to select a row in the memory cell array
80 (e.g., see FIG. 49), while negative voltage is applied to WL
terminals 70b (not shown) through 70n for any unselected rows. The
negative voltage applied reduces the potential of floating body 24
through capacitive coupling in the unselected rows and turns off
the SCR device of each cell 50 in each unselected row. In one
particular non-limiting embodiment, about +0.8 volts is applied to
substrate terminals 78a through 78n, about +0.5 volts is applied to
terminal 70a (for the selected row), and about +0.4 volts is
applied to selected bit line terminal 74a, about -1.0 volts is
applied to unselected word line terminals 70b (not shown) through
70n, and about +0.8 volts is applied to unselected bit line
terminals 74b through 74. However, these voltage levels may
vary.
For memory cells sharing the same row as the selected memory cell
(e.g. cell 50b), both the BL and substrate terminals are positively
biased and the SCR is off. Consequently these cells will not be at
the holding mode. However, because read operation is accomplished
much faster (in the order of nanoseconds) compared to the lifetime
of the charge in the floating body 24 (in the order of
milliseconds), it should cause little disruption to the charge
stored in the floating body.
For memory cells sharing the same column as the selected memory
cell (e.g. cell 50c), the substrate terminal 78 remains positively
biased while the BL terminal 74 is positively biased (at lower
positive bias than that applied to the substrate terminal 78). As
can be seen, these cells will be at holding mode, where memory
cells in state logic-1 will maintain the charge in floating body 24
while memory cells in state logic-0 will remain in neutral
state.
For memory cells sharing neither the same row nor the same column
as the selected memory cell (e.g. cell 50d), both the BL and
substrate terminals are positively biased and the SCR is off.
Consequently these cells will not be at the holding mode. However,
because read operation is accomplished much faster (in the order of
nanoseconds) compared to the lifetime of the charge in the floating
body 24 (in the order of milliseconds), it should cause little
disruptions to the charge stored in the floating body.
The silicon controlled rectifier device of selected memory cell 50a
can be put into a state logic-1 (i.e., performing a write logic-1
operation) as described with reference to FIG. 50. The following
bias is applied to the selected terminals: zero voltage is applied
to BL terminal 74, a positive voltage is applied to WL terminal 70,
and a positive voltage is applied to the substrate terminal 78,
while SL terminal 72 is left floating. The positive voltage applied
to the WL terminal 70 will increase the potential of the floating
body 24 through capacitive coupling and create a feedback process
that turns the SCR device on. Once the SCR device of cell 50 is in
conducting mode (i.e., has been "turned on") the SCR becomes
"latched on" and the voltage applied to WL terminal 70 can be
removed without affecting the "on" state of the SCR device. In one
particular non-limiting embodiment, a voltage of about 0.0 volts is
applied to terminal 74, a voltage of about +0.5 volts is applied to
terminal 70, and about +0.8 volts is applied to terminal 78.
However, these voltage levels may vary, while maintaining the
relative relationships between the voltages applied, as described
above, e.g., the voltage applied to terminal 78 remains greater
than the voltage applied to terminal 74.
For memory cells sharing the same row as the selected memory cell
(e.g. cell 50b), the substrate terminal 78 is positively biased.
However, because the BL terminal 74 is also positively biased,
there is no potential difference between the substrate and BL
terminals and the SCR is off. Consequently these cells will not be
at the holding mode. However, because the write logic-1 operation
is accomplished much faster (in the order of nanoseconds) compared
to the lifetime of the charge in the floating body 24 (in the order
of milliseconds), it should cause little disruption to the charge
stored in the floating body.
For memory cells sharing the same column as the selected memory
cell (e.g. cell 50c), the substrate terminal 78 remains positively
biased while the BL terminal 74 is now grounded. As can be seen,
these cells will be at holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 while memory
cells in state logic-0 will remain in neutral state.
For memory cells not sharing the same row nor the same column as
the selected memory cell (e.g. cell 50d), both the BL and substrate
terminals are positively biased and the SCR is off. Consequently
these cells will not be at the holding mode. However, because the
write logic-1 operation is accomplished much faster (in the order
of nanoseconds) compared to the lifetime of the charge in the
floating body 24 (in the order of milliseconds), it should cause
little disruption to the charge stored in the floating body.
A write logic-0 operation to selected memory cell 50a is described
with reference to FIG. 51. The silicon controlled rectifier device
is set into the blocking (off) mode by applying the following bias:
zero voltage is applied to BL terminal 74a, a positive voltage is
applied to WL terminal 70a, and zero voltage is applied to the
substrate terminal 78, while leaving SL terminal 72a floating.
Under these conditions the voltage difference between anode and
cathode, defined by the voltages at substrate terminal 78 and BL
terminal 74, will become too small to maintain the SCR device in
conducting mode. As a result, the SCR device of cell 50a will be
turned off. In one particular non-limiting embodiment, a voltage of
about 0.0 volts is applied to terminal 74, a voltage of about +0.5
volts is applied to terminal 70, and about 0.0 volts is applied to
terminal 78. However, these voltage levels may vary, while
maintaining the relative relationships between the charges applied,
as described above.
For memory cells sharing the same row as the selected memory cell
(e.g. cell 50b), the substrate terminal 78 is grounded and the SCR
will be off. Consequently these cells will not be at the holding
mode. However, because write operation is accomplished much faster
(in the order of nanoseconds) compared to the lifetime of the
charge in the floating body 24 (in the order of milliseconds), it
should cause little disruption to the charge stored in the floating
body.
For memory cells sharing the same column as the selected memory
cell (e.g. cell 50c), the substrate terminal 78 is positively
biased while the BL terminal 74a is now grounded. As can be seen,
these cells will be at holding mode, where memory cells in state
logic-1 will maintain the charge in floating body 24 while memory
cells in state logic-0 will remain in neutral state.
For memory cells sharing neither the same row nor the same column
as the selected memory cell (e.g. cell 50d), both the BL terminal
74p and substrate terminal 78 are positively biased and the SCR is
off. Consequently these cells will not be at the holding mode.
However, because the write logic-0 operation is accomplished much
faster (in the order of nanoseconds) compared to the lifetime of
the charge in the floating body 24 (in the order of milliseconds),
it should cause little disruption to the charge stored in the
floating body.
While one illustrative embodiment and method of use of the SCR
operation of memory cell 50 has been described, other embodiments
and methods are possible. For example, the first and second
conductivity types may be reversed so that the first conductivity
type is n-type and the second conductivity is p-type making the SCR
a N1-P2-N3-P4 device and reversing the polarity of applied
voltages. Voltages given in the various example operations are
illustrative only and will vary from embodiment to embodiment as a
matter of design choice. While substrate 12 is called a substrate
for continuity of terminology and simplicity of presentation,
substrate 12 may alternately be a well in either another well or a
true substrate in a structure similar to that described in
conjunction with FIG. 9B above. By substrate 12 being a well
instead of a true substrate, manipulating the voltage level of
substrate 12 as required in some SCR operations is facilitated.
Many other alternative embodiments and methods are possible, thus
the illustrative examples given are not limiting in any way.
A novel semiconductor memory with an electrically floating body
memory cell is achieved. The present invention also provides the
capability of maintaining memory states employing parallel
non-algorithmic periodic refresh operation. As a result, memory
operations can be performed in an uninterrupted manner. Many
embodiments of the present invention have been described. Persons
of ordinary skill in the art will appreciate that these embodiments
are exemplary only to illustrate the principles of the present
invention. Many other embodiments will suggest themselves to such
skilled persons after reading this specification in conjunction
with the attached drawing figures. For example:
The first and second conductivity types may be reversed and the
applied voltage polarities inverted while staying within the scope
of the present invention.
While many different exemplary voltage levels were given for
various operations and embodiments, these may vary from embodiment
to embodiment as a matter of design choice while staying within the
scope of the present invention.
The invention may be manufactured using any process technology at
any process geometry or technology node and be within the scope of
the invention. Further, it should be understood that the drawing
figures are not drawn to scale for ease of understanding and
clarity of presentation, and any combination of layer composition,
thickness, doping level, materials, etc. may be used within the
scope of the invention.
While exemplary embodiments typically showed a single memory array
for the purpose of simplicity in explaining the operation of the
various memory cells presented herein, a memory device employing
the memory cells of the presentation may vary in many particulars
in terms of architecture and organization as a matter of design
choice while staying within the scope of the invention. Such
embodiments may, without limitation, include features like, for
example, as multiple memory arrays, segmentation of the various
control lines with or without multiple levels of decoding,
simultaneously performing multiple operations in multiple memory
arrays or in the same arrays, employ many different voltage or
current sensing circuits to perform read operations, use a variety
of decoding schemes, use more than one type of memory cell, employ
any sort of interface to communicate with other circuitry, and
employ many different analog circuits known in the art to generate
voltage or currents for use in performing the various operations on
the memory array or arrays. Such analog circuits may without
limitation be, for example, digital-to-analog converters (DACs),
analog-to-digital converters (ADCs), operational amplifiers (Op
Amps), comparators, voltage reference circuits, current mirrors,
analog buffers, etc.
Thus the invention should not be limited in any way except by the
appended claims.
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